Studies on metabolism of vitamin A. Absorption of retinal (vitamin A aldehyde) in rats

1. Young rats with low reserves of vitamin A were dosed with retinal in groundnut oil, and the stomach, the contents, mucosa and muscles of small intestine, the blood and the liver were analysed at periods up to 24hr. after dosing. 2. Up to 6hr. after the dose, retinal was present in high concentrations in the contents, mucosa and muscles of the intestine. Small but significant amounts were present in blood and liver at all times. 3. The intestinal mucosa and muscles always contained small amounts of retinol and its esters. 4. A study of the distribution of the three forms of the vitamin within the mucosal cell showed that most of the mucosal retinal enters the cell unchanged. 5. When protein-depleted rats were similarly given retinal, the rate of reduction of the aldehyde, and the consequent deposition in the liver of retinol and its esters, progressively decreased with reduced protein intake.     

Dietary protein intake and skeletal-muscle protein metabolism in rats. Studies with salt-washed ribosomes and transfer factors

1. Aspects of skeletal muscle protein synthesis in vitro were studied in young rats given a low-protein diet for up to 10 days and during re-feeding with an adequate diet. 2. Partially purified muscle transfer factors (transferases I and II), crude and purified (NH(4)Cl-washed) ribosomes and a pH5 enzyme fraction were prepared for this purpose. 3. A marked decrease in the capacity of crude ribosomes to carry out cell-free polypeptide synthesis occurred within 4 days of feeding the low-protein diet. 4. The capacity of salt-washed ribosomes to promote amino acid polymerization, in the presence of added transfer factors and aminoacyl-tRNA, was only slightly decreased by the dietary treatment. 5. However, the capacity of salt-washed ribosomes to bind (14)C-labelled aminoacyl-tRNA was decreased by feeding the low-protein diet. 6. The capacity of the pH5 enzyme fraction to promote amino acid incorporation in a complete cell-free system was decreased within 2 days of feeding the low-protein diet. There is no evidence that the change is associated with aminoacyl-tRNA synthetase or binding enzyme activities of the pH5 fractions. 7. These changes are discussed in relation to the diminished rate of protein synthesis in the intact muscle cell when rats are given a low-protein diet.     

Studies of vitamin A metabolism in mouse model systems

Over the past several years, discoveries from mouse genetics have had direct impact on our understanding of vitamin A metabolism. Although the metabolism of vitamin A in the mouse does have some special features (for example very large stores of liver and pulmonary retinyl esters), the ability to construct knockout and transgenic mouse models has yielded an impressive amount of information directly relevant to understanding the general principles of vitamin A transport, storage and degradation. We discuss below the metabolism of vitamin A through a number of genetically engineered mouse strains with alterations in genes that affect this metabolism. The novelty of this experimental approach is evidenced by the fact that the oldest of these strains was first reported only eight years ago.1) Copyright 2001 John Wiley & Sons, Inc.     

Studies on metabolism of vitamin A. The effect of the stage of vitamin A deficiency on sulphate activation in rat liver

1. The synthesis of ;active sulphate' in rat liver was studied at various stages of vitamin A deficiency, with the corresponding pair-fed controls. 2. The activity was significantly decreased even at the onset of the deficiency, and at the acute stage there was further loss. 3. Only at the earlier stages of the deficiency was the addition of retinol, in vitro, fully effective in restoring the lost activity; retinoic acid was partially active. No such restoration was possible at the acute deficiency stage.     

Studies on metabolism of vitamin A. The effect of hormones on gestation in retinoate-fed female rats

1. The preparation of cell suspensions by treatment of chick embryo hearts with collagenase at various stages of development is described. 2. Measurements of oxygen consumption, incorporation of labelled leucine into protein and accumulation of labelled alpha-aminoisobutyric acid against a concentration gradient indicated a long-lasting viability of the isolated heart cells in vitro; a satisfactory preservation of subcellular structures, including plasma membrane, was assessed by electron-microscopic examination. 3. The rate of alpha-aminoisobutyric acid accumulation by cardiac cells isolated from hearts at different stages of embryological development decreased with aging; insulin stimulated the intracellular accumulation of this amino acid analogue. 4. Insulin increased the uptake by isolated heart cells of several (14)C-labelled naturally occurring amino acids; however, the fraction of amino acid taken up by the cells that was recovered free intracellularly, and therefore the concentration ratio (between intracellular water and medium), was enhanced by the hormone only with glycine, proline, serine, threonine, histidine and methionine. When isolated heart cells were incubated in the presence of a mixture of labelled amino acids, the addition of insulin increased the disappearance of radioactivity from the medium. 5. The general pattern of amino acid transport (in the absence and in the presence of insulin) in isolated cardiac cells was similar to that found in intact hearts, suggesting that the biological preparation described in this paper might be useful for studies of cell permeability and insulin action.