Abstract28-day-old weanling rats were fed a diet containing 3% casein as the only source of protein for eight weeks to induce protein deficiency. When compared to control animals (fed a diet containing 25% casein), these rats had significantly lowered body (5.2-fold reduction) and liver (2.5-fold reduction) weights. The circulatory level of retinol (nmoi per ml plasma) as well as retinol (nmol per g tissue) in the liver of these protein-deficient animals were also reduced significantly, although their liver concentration of retinyl palmitate (nmol per g tissue) was comparable to that of the control group. Assay of liver tissue for retinyl palmitate hydrolase activity revealed a 4-fold reduction (compared to that of control animals) of specific enzyme activity (nmol retinol formed per g protein per h). These findings suggest that severe protein deficiency results in a decreased hydrolysis of retinyl esters in the liver, which may be in part responsible for the reduced level of metabolically 'active' retinoids available for normal physiological functions.

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Goldfish with retinas rich in either rhodopsin or porphyropsin were illuminated with bright light and then placed in the dark room to allow visual pigment regeneration. The kinetics of this in vivo pigment regeneration were followed by sampling these animals at regular time intervals. The first-order kinetic rate constant for the initial period of porphyropsin regeneration at 20 degrees C was 8.3 X 10(−3) nmol kg-1 body weight min-1 and the half-life of this reaction was 83 min. At 30 degrees C, the rate constant was increased to 1.4 X 10(−2) nmol kg-1 body weight min-1, yielding a reduced half-life of 49 min. This suggests that the Q10 of porphyropsin regeneration is about 1.7. In goldfish retinas enriched with rhodopsin (62% rhodopsin and 38% porphyropsin), the initial phase of visual pigment regeneration (at 30 degrees C) proceeded at a slower rate (first-order rate constant: 6.5 X 10(−3) nmol kg-1 body weight min-1; half-life of reaction = 106 min) than the rate of porphyropsin regeneration. This suggests that the high proportion of rhodopsin in the retina of goldfish held at 30 degrees C is not a direct result of a faster rate of regeneration of rhodopsin than of porphyropsin.

Using high-performance liquid chromatography, the quantity of retinol, retinal, and retinyl esters as well as their 3,4-didehydro derivatives were measured in several body compartments and in the remainder of the body of 11 goldfish. Eighty-six percent of these retinoids and 3,4-didehydroretinoids existed as retinyl and 3,4-didehydroretinyl esters, 10% as retinal and 3,4-didehydroretinal, and 4% as retinol and 3,4-didehydroretinol. Most (94%) of these retinoids and 3,4-didehydroretinoids were found in ocular tissues and high ratios of 3,4-didehydroretinoids/ retinoids were observed in most tissue extracts. These data suggest that in the goldfish ocular tissues are important locations of retinoid and 3,4-didehydroretinoid metabolism, which favors the formation and (or) storage of 3,4-didehydroretinoids.