Volume 15 Number 1 Reports 47 ditions of constant dark was readily apparent. Norepinephrine levels in iris and ciliary processes after both experimental procedures, moreover, were no different from those measured under control conditions. All methods previously used for producing disuse supersensitivity share the common property of chronic interruption of the normal contact between a neurotransmitter and its effector cells. 2 The supersensitivity observed in the iris dilator after constant light is accordingly assumed to be due to the removal of a trophic influence of the normal transmitter, norepinephrine, rather than its absolute loss. The failure for subsensitivity of the dilator to develop in response to over-stimulation by constant darkness, on the other hand, may indicate that the activity and sensitivity of this muscle is normally quite pronounced. Supersensitivity of the iris dilator after conditions of constant light became manifest in response to the alpha agonist norepinephrine, the mixed alpha- and beta-agonist epinephrine, and the beta-agonist isoproterenol. Since pupillary dilation in response to "pure" beta-agonists is currently thought to be mediated by stimulation of alphareceptors in the iris dilator, 9 it seems logical to assume that a common underlying mechanism is involved in producing the observed supersensitivity to all three sympathomimetics. Most of the studies on the phenomena of supersensitivity and subsensitivity of excitable tissues have necessarily utilized marked surgical or pharmacological intervention. 1-2 The present experiments demonstrate that sensitivity changes in the iris dilator can be induced much less drastically by variation of the physiologic stimulus, light. Use of these experimental conditions should thus provide an excellent model for study of the interaction of adrenergic and cholinergic nervous systems in ocular function. The authors acknowledged with gratitude Dr. William W. Fleming for his constructive advice during the course of these experiments. The technical assistance of Ms. Lisa Gustafson and Ms. Jane Kosa is also gratefully acknowledged. From the Departments of Surgery (Ophthalmology) and Pharmacology, West Virginia University Medical Center, Morgantown, W. Va. 26506. Supported by grants from the National Eye Institute (EY-01217) and the West Virginia University Medical Corporation (0216.080-002). Submitted for publication May 28, 1975. Key words: iris dilator, norepinephrine, mydriatics, lighting conditions, supersensitivity. REFERENCES 1. Trendelenburg, U.: Supersensitivity and subsensitivity to sympathomimetic amines, Pharmacol. Rev. 15: 225, 196. 2. Fleming, W. W., McPhillips, J. J., and Westfall, D. P.: Postjunctional supersensitivity and subsensitivity of excitable tissues to drugs, Ergeb. der Physiol. 68: 56, 197.. Smith, A. A., and Dancis, J.: Physiologic studies in familial dysautonomia, J. Pediatr. 6: 88, 196. 4. Sears, M. L., Kier, E. L., and Chavis, R. M.: Homer's syndrome caused by occlusion of the vascular supply to sympathetic ganglia, Am. J. Ophthalmol. 77: 717, 1974. 5. Bito, L. Z., Hyslop, K., and Hyndman, J.: Antiparasympathomimetic effects of cholinesterase inhibitor treatment, J. Pharmacol. Exp. Ther. 157: 159, 1967. 6. Bito, L. Z., Dawson, M. J., and Petrinovic, L.: Cholinergic sensitivity: normal variability as a function of stimulus background, Science 172* 58 1971 7. Fleming,' W. W., Wesrfall, D. P., De La Lande, I. S., et al.: Lognormal distribution of equieffective doses of norepinephrine and acetylcholine in several tissues, J. Pharmacol. Exp. Ther. 181: 9, 1972. 8. Bertler, A., Carlsson, A., and Rosengren, E.: A method for the fluorometric determination of adrenaline and noradrenaline in tissue, Acta Physiol. Scand. 44: 27, 1958. 9. Langham, M. E., Simjee, A., and Josephs, S.: The alpha and beta adrenergic responses to epinephrine in the rabbit eye, Exp. Eye Res. 15: 75, 197. Retinal degeneration in cats fed casein. I. Taurine deficiency. SUSAN Y. SCHMIDT, ELIOT L. BERSON, AND K. C. HAYES. All cats fed a taurine-free casein diet for at least 2 weeks have shown granularity with a hyper-reflective white zone in the area centralis, nondetectable electroretinograms (ERG's), and structural changes indicating photoreceptor cell degeneration. The present study has demonstrated that cats fed this casein diet have a selective decrease in plasma and l taurine concentrations by five weeks; taurine levels were about 4 per cent of normal in plasma, and 60 per cent of normal in. After 10 weeks, taurine levels were 2 to 4 per cent of normal in plasma and reached a minimum of 20 to 0 per cent of iwrmal in the. These biochemical changes occurred in association with a delay in the cone ERG implicit time at five weeks and- reduced cone and rod ERG amplitudes at 10 weeks. During this period, l DNA content (as a measure of cell viability) and fundus appearance were normal. By 2 weeks, ERG's were nondetectable, l DNA content was reduced, and the fundus showed typical changes in the area centralis. These studies help to establish a biological role for taurine in maintaining photoreceptor cell function and viability in the cat.
48 Reports Investigative Ophthalmology January 1976 Table I Ingredients Casein (vitamin-free)* Dextrin Sucrose Safflower oil Salt mixf Vitamin mix$ Alpha-tocopheryl acetate Vitamin A palmitate beadlets (IU) Choline chloride Protein (% of calories) Grams per 100 Gm. 6.0 4.5 10.0 15.0 4.0 0.2 0.005 4,000 0. 26.8 "General Biochemicals, Chagrin Falls, Ohio. The casein contained 86% protein by analysis. The casein diet contained 0.5 Gm. methiqnine per 100 Gm. (comparable to chow), 0.1 Gm. cysteine per 100 Gm. (chow; 0.8 Gm. per 100 Gm.), 1.1 Gm. serine per 100 Gm. (comparable to chow) and no taurine (chow; 0.1 Gm. per 100 Gm.). fthe salt mix contained (in grams per kilogram): calcium carbonate, 00; potassium phosphate dibasic, 22; calcium phosphate dibasic, 75; magnesium sulfate, 102; sodium chloride, 167; ferric citrate, 27.50; potassium iodide, 0.80; manganese sulfate, 5.00; zinc chloride, 0.25; cupric sulfate, 0.0; chromium acetate, 0.0458; sodium selenite, 0.0044. (The vitamin mix contained: inositol, 60 Gm.; niacin, 8 Gm.; para-aminobenzoic acid, 8 Gm.; calcium pantothenate, 4 Gm.; riboflavin, 1.6 Gm.; pyridoxine HC1, 800 mg.; folic acid, 200 mg.; thiamin HC1, 800 mg.; biotin, 40 mg.; menadione, 200 mg.; cyanocobalamin, 10 mg.; vitamin D^ beadlets, 400,000 IU, made to 00 Gm. with dextrin. Taurine has been found in high concentrations in the s of all species studied; the level exceeds by at least threefold that of any other free amino acid. 1 " 5 However, its biological role remains unknown. A severe deficiency of plasma and l taurine has been demonstrated in cats fed casein as the only source of dietary protein. By 2 weeks, these cats showed granularity in the area centralis with a central, highly reflective white zone and had nondetectable electroretinograms (ERG's). Structural changes in the photoreceptor cells were greatest in the area centralis at 2 weeks, and many photoreceptor cell nuclei in central and peripheral were lost within one year. 2 - G - The present investigation was done to follow the development of plasma and l taurine deficiency in cats fed casein for up to 2 weeks. The concentrations of taurine and other free amino acids were measured in the area centralis and peripheral. Retinal function was monitored with the ERG, and l DNA content was used as an index of l cell viability. Materials and methods. Twenty-six adult domestic cats each weighing about 2 kilograms were fed a taurine-free casein diet (Table I) or commercial chow (Ralston Purina) that contains taurine. Fourteen cats were fed the casein diet for five to 2 weeks while eight others were fed chow during the same period. Water and food were provided ad libitum, and food intake was monitored daily to ascertain that all animals were eating their respective diets. To control for reduced food intake in the casein-fed cats, four cats were pair-fed an amount of chow equal to the quantity of casein diet consumed by four of the casein-fed cats. The cats were individually housed in a room maintained at 25 C. under cyclic light (12 hours on/12 hours off), and the average cage illumination was eight footcandles. 55 ERG testing and fundus examinations were done prior to the start of these experiments and after 5, 10, 15, and 2 weeks of casein or chow diet. ERG's were recorded to single flashes of white light under conditions of dark adaptation or to a 40 cps. white flickering stimulus in a full-field, Ganzfeld system as described previously. 8 This white light flash elicited 400 MV ± 65 MV responses (mean t S.D.) from a normal dark-adapted cat and 15 nv ± 2 /xv (mean ± S.D.) responses under conditions of light adaptation. Therefore, under conditions of dark adaptation, the response to white light was rod-dominated (95 per cent), assuming linear summation of cone and rod system components. In this study, implicit times were defined as the time intervals between stimulus onset and major cornea-negative (a-wave) or cornea-positive (b-wave) peaks of the responses. After 2, 5, 10, 15, and 2 weeks of chow or casein diet, blood was collected from cats fasted for 16 hours, and the plasma fraction was deproteinized with sulfosalicylic acid (0 mg. per milliliter) according to the method of Perry and Hansen. 9 At the same time intervals, cats were anesthetized (sodium pentobarbital 0 mg. per kilogram), and the eyes were removed. Small sections of the from the area centralis, peripheral-tapetal, and peripheral-nontapetal regions were separated from the pigment epithelium, rapidly frozen in dry ice within five minutes, and lyophilized. Freeze-dried samples weighing approximately 0.5 to 1 mg. were homogenized in water and deproteinized with sulfosalicylic acid. Amino acids in the deproteinized supernatants of and plasma were determined on a Beckman Model 121 automatic amino acid analyzer. The extracts were diluted 1:1 with sodium citrate buffer (0.2M, ph.25), and a 0.25 ml. aliquot was placed on a single long column of Beckman AA-15 resin. The amino acids were eluted during a -hour run with 0.2 M sodium citrate buffers (ph.25 followed by ph 4.25) at a temperature of 52.5 C., at a flow rate of 70 ml. per hour. DNA was determined in aqueous homogenates of freeze-dried s according to the fluorometric micromethod of Kissane and Robins. 10 Results. Table II shows comparable reductions in taurine concentrations in three regions of the from cats fed casein for 5, 10, and 2 weeks. After five weeks of casein diet, the average l taurine level was 64 per cent of that seen
Volume 15 Number 1 Reports 49 Table II. Amino acid concentration of three regions of the from chow-fed and casein-fed cats Amino acid (nmoles/mg. dry wt) Taurine Aspartic acid Glutamine + serine Glutamic acid Glycine Alanine Number of samples Normal cats fed chow Area tapetal centralis 160 ± 1 170 ± 7 7± 1 7± 1 15 ±1 1 ±1 26 ± 2 24 ± 2 9 ± 0.4 7 ± 0.4 ± 0.4 2 ± 0.4 9 9 /0 weefcy casein non tapetal 177 ± 21 12 ± 1 27 ±2 5± 1 1 ± 1 Area centralis 98 ±6* 17 ± 1 29 ±2 10 ± 1 ± 1 10 5 weeks casein tapetal 110± 11* 16 ±2 27 ± 1 8 ±0.4 4 ±0. 8 2 weeks casein tapetal 42 ± 1* 19 ± 22 ± 1 8± 1 ± 1 nontapetal 125 ± 5*' 5± 2 19 ± 5 2 ± 10 ±0. ± 1 Area centralis tapetal 51 ±8* 50 ±10* 10 ± 1 8± 1 28 ± 4 22 ± ± 0 ± 11 ± 1 8± 1 5 ± 0. 4 ± 0.4 5 4 Values represent the mean ± S.E.M. 'Significantly different from normal (p < 0.01). nontapetal 60 ±9* 9± 1 26 ±6 ± 10 ± 1 4 ±0. Area centralis 0 ±6* 9± 1 1 ±5* 28 ±4 1± 1* 5± 1 4 nontapetal 49 ± 19* 7± 2 19±4 22 ± 9± 1 2± 1 Table III. Plasma amino acids in chow-fed and casein-fed cats Amino acid (nmoles/ml.) Taurine Cysteine Methionine Glutamic acid Serine + glutamine Glycine Alanine Aspartic acid Number of samples Chow-fed (normal) 109 ± 18 75 ±2 66 ± 8 72 ± 19 1,170 ± 70 11 ± 24 561 ± 78 8± 1 8 The values represent the mean ± S.E.M. Casein-fed 2 weeks 5 weeks 10 weeks 2 weeks 0 ± 18 95 ± 12 7 ± 1 62 ±5 1,418 ± 78 8 ± 72 484 ± 29 12 ±4 2 2± 1 62 ±8 78 ± 10 44 ±6 1,0 ±64 41 ±46 665 ± 64 6 ± 1 7 4±2 86 ±5 62 ±5 88 ± 10 1,20 ± 54 75 ± 0 652 ± 46 9±2 6 1 ± 1 6 ± 12 68 ± 17 85 ± 14 1,0 ± 19 25 ± 29 00 ± 17 12± 4 in normal cats fed chow. Retinal taurine levels were, respectively, 1 per cent and 24 per cent of normal for cats fed casein for 10 and 2 weeks. After 2 weeks of casein diet, glutamine plus serine and glycine concentrations were significantly higher than normal in the area centralis. The concentrations of other free amino acids in central and peripheral s of casein-fed cats remained the same as those in the control cats. Table III shows that plasma taurine levels decreased rapidly while plasma methionine and cysteine remained normal in casein-fed cats. Other plasma free amino acids (known to be present in the ) also remained normal in the plasma of casein-fed cats. Fig. 1 demonstrates that plasma taurine decreased more rapidly than l taurine. By 10 weeks both l and plasma taurine reached their lowest levels, while DNA concentrations in central and peripheral l areas remained normal (105 ± 7 /ug per milligram dry weight, mean ± S.E.M.). At 2 weeks, l DNA concentration was reduced (84 ± 2 fig per milligram dry weight). After cats were fed casein for five weeks, their cone ERG implicit times (Fig. 2) were 25.4-0.7 msec, (mean ± S.E.M.) compared with normal cone implicit times of 22.0 ± 0.4 msec. This difference (15 per cent) was significant (two-tailed Student's t-test, p < 0.002) while the implicit times of the dark-adapted a-wave and b-wave responses were not significantly different from normal. After 10 weeks of casein diet, the darkadapted a-wave was minimally, if at all, delayed while the b-wave responses were clearly delayed.
50 Reports Investigative Ophthalmology January 1976 140 1OOr 10 40 cps 2 80 60 - Retinal DNA 120 b- wave O 40 110 20 Retinal Taurine ^ 100 o cc 0 2 5 10 15 WEEKS ON CASEIN DIET 20 2 Plasma Taurine Fig. 1. Retinal DNA ( ), l taurine (O), and plasma taurine (#) concentrations in cats fed casein for up to 2 weeks. Values for each cat at 5, 10, 15, and 2 weeks were compared to those obtained from the same cat prior to feeding casein. In chow-fed controls, average l concentration of DNA is 106 ± 5 Mg per milligram of dry weight; average l taurine is 166 ± 7 nmoles per milligram of dry weight, and average plasma taurine is 109 ± 18 nmoles per milliliter. The values represent the mean ± S.E.M. for to 10 determinations. 0 5 WEEKS ON CASEIN DIET Fig. 2. Measurements of ERG implicit time for cats fed casein for 10 weeks. Cone responses (#) were isolated with a 40 c.p.s. flickering white light stimulus. The a-wave (O) and b-wave ( ) implicit times were obtained from rod dominated responses to white light in the dark-adapted state. Implicit times for each cat at 5 and 10 weeks were expressed as percentages of the implicit times obtained from the same cat prior to feeding casein. Each symbol represents the mean ± S.E.M. for responses from 6 to 10 cats. 10 ERG amplitudes were within the normal range after five weeks of casein diet but were significantly reduced by 10 weeks (Fig. ). In contrast to the early ERG changes, findings on ophthalmoscopic examination were relatively minimal. Granularity in the area centralis was the only visible abnormality after 15 weeks of casein diet. By the twenty-third week, a hyperreflective spot had developed in the center of the area centralis surrounded by a zone of granularity. All cats fed chow retained a normal appearance to the fundus and had normal ERG's. Reduced food intake was observed in cats fed casein. Average weight of the casein-fed cats was 2.11 Kg. ± 0.16 (S.E.M.) prior to the start of the experiment and did not change during these experiments (2.12 Kg. ± 0.07 after 10 weeks and 2.2 Kg. ± 0.12 at 2 weeks). The chow-fed cats weighed 1.94 Kg. ± 0.22 initially and did gain weight over the 2-week period (.00 Kg. ± 0.19 after 10 weeks and.80 Kg. ± 0.25 at 2 weeks). The four adult controls that were pair-fed to four of the casein-fed cats had weights comparable to those of the casein-fed cats and had normal fundus examinations and normal ERG's. Discussion. Recent observations have shown that taurine is in high concentration in the photoreceptor cell layer of normal frog and mouse 1 s. In the CH mouse with l dystrophy, Cohen, McDaniel, and Orr 1 demonstrated that l levels are about 0 per cent of normal 100 80 60 40 g 20 I I 40 cps TO a-wav< b-wave 0 5 10 WEEKS ON CASEIN DIET Fig.. Measurements of ERG amplitude for cats fed casein for 10 weeks. Amplitudes for each cat at 5 and 10 weeks were expressed as percentages of the amplitudes obtained from the same cat prior to feeding casein. Each response represents the mean ± S.E.M. for responses from 6 to 10 cats. after all photoreceptor cells have degenerated. Retinal DNA content (micrograms per milligram of dry weight) in the CH mouse at advanced stages of photoreceptor degeneration is about 50 per cent of normal. 11 In the present investigation, after 10 weeks of casein diet, l taurine levels
Volume 15 Number 1 Reports 51 TAURINE METABOLISM METHIONINE CHJ-S-CHJ-CHJ-CH-COOH NH, -CH, Homocysteine HS-CH 2-CH 2-CH-COOH NH, Cystathionine SERINE CH 2-CH-COOH OH NH 2 2 ATP + SO* = Phosphoadenosine Phosphosulfate (PAPS) Coenzyme A u* HjS+NHa* _ ICYSTEINEl ^ Pyruvate PAPS Transferase B6 Cysteine Sulfinic Acid -co 2 Bfi [O] Cysteic Acid O NH 2 HO-S-CH 2-CH-COOH I-CO, B6 Cysteamine HS-CH 2-CH 2-NH 2 [O] Hypotaurine HO-S- CH,- CH,- NH, 2 2 2 o [O] TAURINE o HO-S-CH,-CH,-NH, i 2 II * O I-NH, Isethionic Acid O HO- -CH 2-CH 2-OH O Fig. 4. Taurine metabolism. 11 " 17 Amino compounds (capitalized) were measured in the plasma and of chow- and casein-fed cats. were low and ERG's were reduced 50 to 70 per cent below normal when l DNA levels were normal; these observations support the idea that photoreceptor cell function is altered by l taurine deficiency prior to photoreceptor cell death. Subsequent experiments 0 have shown that supplementation of the casein diet with taurine maintained photoreceptor function, further supporting the idea that taurine is important for photoreceptor cell viability in the cat. The fall in l taurine content was associated with prolonged cone ERG implicit times and then with reduced ERG amplitudes. The delays in b-wave implicit times raise the possibility of an effect of taurine deficiency on intral transmission of photoreceptor responses or on Miiller cell function. However, no structural alterations were demonstrated in the early stages of this degeneration in the photoreceptors at the site of contact with proximal l cells or in the cells of the inner nuclear layer. 7 Cells of the inner contain relatively high concentrations of glycine and glutamine plus serine. 1 The preservation of the inner with loss of photoreceptor cells would be consistant with the observed rise in glycine and glutamine plus serine in the area centralis in cats fed the casein diet for 2 weeks (Table II). It was interesting to note that the photoreceptor abnormalities were initially greatest in the area centralis 7 (region of maximal cone concentration) 12 even though reductions of l taurine concentration were similar throughout the central and peripheral. The striking decrease in l and plasma taurine levels in cats fed the casein diet occurred while the levels of taurine precursors, methionine, cysteine, and serine, remained normal in plasma.- Since the casein diet contained normal amounts of methionine and serine, small amounts of cysteine and no taurine (Table I), the plasma values suggested that the casein-fed cats could utilize methionine and serine to synthesize cysteine but could not efficiently convert cysteine to taurine (Fig. 4). Since the normal cat (as well as man) has a low level of liver decarboxylase activity, it is probable that cysteine metabolites (cysteine sulfinic acid and cysteic acid) are converted to taurine at a slow rate. Another factor that could contribute to l taurine deficiency
52 Reports Investigative Ophthalmology January 1976 is based on the observation that the casein diet is low in inorganic sulfate and that an alternate pathway for taurine synthesis in the liver of chick and rat, 14 and possibly the cat, 15 involves the fixation of inorganic sulfate with serine. Again the liver decarboxylase could be limiting in the conversion of cysteic acid to taurine. The reason for disruption of outer segment structure and photoreceptor cell death in the taurine-deficient cat is not known. Young 18 has shown that taurine-h is initially concentrated in the pigment epithelium and then distributed throughout the photoreceptor cells of rats and frogs. On the other hand, Ehinger 19 has observed that the pattern of distribution of taurine-h follows that of the Miiller cell in the rabbit. Studies with labeled taurine are in progress in the taurine-deficient cat to learn more about the pathogenesis of this photoreceptor cell degeneration. The authors thank Gail Watson and Caecilia Huang for technical assistance and Jennifer Moulton for typing the manuscript. From the Berman-Gund Laboratory for the Study of Retinal Degenerations, Harvard Medical School, Massachusetts Eye and Ear Infirmary and the Department of Nutrition, Harvard School of Public Health. This work was supported by grants from the National Retinitis Pigmentosa Foundation, Baltimore, Md.; the George Gund Foundation, Cleveland, Ohio; Research Grants EY00169 and EY0061 and Career Development Award EY70800 (ELB) from the National Eye Institute; and the Fund for Research and Teaching, Department of Nutrition, Harvard School of Public Health. Submitted for publication June 17, 1975. Reprint requests: Dr. S. Y. Schmidt, Berman- Gund Laboratory, 24 Charles St., Boston, Mass. 02114. Key words: l degeneration, taurine, electroretinogram,, cat, diet, casein, amino acids. REFERENCES 1. Cohen, A. I., McDaniel, M., and Orr, H.: Absolute levels of some free amino acids in normal and biologically fractionated s, INVEST. OPHTHALMOL. 12: 686, 197. 2. Hayes, K. C, Carey, R. E., and Schmidt, S. Y.: Retinal degeneration associated with taurine deficiency in the cat, Science 188: 949, 1975.. Kennedy, A. J., and Voaden, M. J.: Free amino acids in the photoreceptor cells of the frog, J. Neurochem. 2:109, 1974. 4. Kubicek, R., and Dolenek, A.: Taurine et acides amines dans la retine des animaux, J. Chromatogr. 1: 266, 1958. 5. Pasantes-Morales, H., Klethi, J., Ledig, M., et al.: Free amino acids of chicken and rat, Brain Res. 41: 494, 1972. 6. Berson, E. L., Hayes, K. C, Rabin, A. R., et al.: Retinal degeneration in cats fed casein: II. Supplementation with methionine, cysteine or taurine, INVEST. OPHTHALMOL. 15: 52, 1975. 7. Hayes, K. C, Rabin, A. R., and Berson, E. L.: An ultrastructural study of nutritionally induced and reversed l degeneration in cats, Am. J. Pathol. 78: 505, 1975. 8. Rabin, A. R., Hayes, K. C, and Berson, E. L.: Cone and rod responses in nutritionally induced l degeneration in the cat, INVEST. OPHTHALMOL. 12: 694, 197. 9. Perry, T. L., and Hansen, S.: Technical pitfalls leading to errors in the quantitation of plasma amino acids, Clin. Chim. Acta 25: 5, 1969. 10. Kissane, J. M., and Robins, E.: The fluorometric measurement of deoxyribonucleic acid in animal tissues with special reference to the central nervous system, J. Biol. Chem. 2: 184, 1958. 11. Lolley, R. N.: RNA and DNA in developing e: comparison of a normal with the degenerating e of CH mice, J. Neurochem. 20: 175, 197. 12. Steinberg, R. H., Reid, M., and Lacey, P. L.: The distribution of rods and cones in the of the cat (Felis domesticus), J. Comp. Neurol. 148: 229, 197. 1. Jacobsen, J. G., and Smith, L. H., Jr.: Biochemistry and physiology of taurine and taurine derivatives, Physiol. Rev. 48: 429, 1968. 14. Martin, W. G., Truex, C. R., Tarka, S. M., et al.: The synthesis of taurine form sulfate. VIII. A constitutive enzyme in mammals (887), Proc. Soc. Exp. Biol. Med. 147: 56, 1974. 15. Rambaut, P. C, and Miller, S. I.: Studies of sulfur amino acid nutrition in the adult cat, Fed. Proc. 24: 7, 1965. 16. White, A., Handler, P., and Smith, E. L.: Principles of Biochemistry, Ed. 5. New York, 197, McGraw-Hill, pp. 681-686. 17. Yamaguchi, K., Shigehisa, S., Sakakihara, S., et al.: Cysteine metabolism in vivo of vitamin Bo-deficient rats, Biochim. Biophys. Acta 81: 1, 1975. 18. Young, R. W.: The organization of vertebrate photoreceptor cells, m: The Retina: Structure, Function and Clinical Characteristics, Straatsma, B., Allen, R., Hall, M., et al., editors. Berkeley, 1969, University of California Press, pp. 177-209. 19. Ehinger, B.: Glial uptake of taurine in the rabbit, Brain Res. 60: 512, 1971. Retinal degeneration in cats fed casein. II. Supplementation with methionine, cysteine, or taurine. ELIOT L. BERSON, K. C. HAYES, ARNOLD R. RABIN, SUSAN Y. SCHMIDT, AND GAIL WATSON. All cats fed a taurine-free casein diet for 2 weeks have shown a nondetectable electro-