Vitamins detailed experiments

In turning our attention to vitamin B12 and its coenzymes, it should be recognized that present diflFerences in opinions and in the interpretations oflFered on the mechanism of coenzyme Bi2-catalyzed processes are largely a consequence of the complexity of these reactions and the difficulty of interpreting the results of enzymological studies in the absence of empirical data derived from model experiments. The situation is best exemplified by first considering an early mechanism postulated for the function of coenzyme B12 in diol dehydratase, not supported by model studies. Ref. 6 contains a detailed discussion. 

Nicolaysen and Jansen (1939) in a series of varied experiments reached the conclusion that vitamin D influences the structure, viz., the matrix of the bones. This work will be reviewed in some detail. Table III, which is condensed from their publication, is instructive as regards the variations in dietary Ca and P. The main argument was that any possible effect of vitamin D on the bones, in addition to the indirect one following defective absorption, could be revealed only if the vitamin D-free and the vitamin D-treated rats received an equal supply of calcium and phosphate into the blood stream. 

Apart from that of E. coli, the only microbial system for deoxyribonu-cleotide synthesis which has been studied in detail is that of L. leichmannii. Prior to the definitive biochemical studies described below, nutritional experiments had demonstrated that the vitamin Bu requirement of L. leichmannii was involved in an essential way in the biosynthesis of deoxyribonucleotides. Blakley and Barker 24) showed that cell-free extracts of this microorganism catalyzed the reduction of cytidylate to deoxycyti-dylate and showed also that this reaction required a vitamin B12 derivative and NADP. 

On the basis of studies of the effect of excessive doses of vitamin A on culture bones and on the tail of Xenopus laevis, a theory has been put forward suggesting that vitamin A acts by rupturing lysosomes and by releasing hydrolytic enzymes in the media. In vitro experiments done in Dingle s laboratory [112, 113] and in De Duve s laboratory [114], where the hydrolases were released under the influence of vitamin A, are often called upon to support this theory. This theory meets with a number of serious objections (1) the release of a large number of hydrolases can hardly be reconciled with the effect of the vitamin on mucus synthesis and steroid hormone biosynthesis and (2) the experiments done in vivo on the Xenopus laevis were carried out with doses of vitamin A that killed 30% of the larvae, so that much of the effect may be due to necrosis rather than to a specific effect of the vitamin. Similar objections can be made with respect to experiments done in vitro with organ culture. The validity of the interpretation of an effect of vitamin A on the isolated particles is discussed in more detail in the section on cellular necrosis. 

The structure of p-carotene had been determined as early as 1930 (Karrer et ai). These two results made it possible to see from a chemical standpoint why p-carotene is a natural precursor of vitamin A compounds, a fact that had already been discovered between 1928 and 1930 in a large number of animal experiments (von Euler et al., 1929 Moore, 1930). The mechanism of the conversion of P carotene to retinaldehyde (2) was explained in detail only at a much later date (Goodman and Olson, 1969). 

Hertz has investigated in great detail the effect of vitamin C deficiency on the regeneration of bone in the fractured fibula of a guinea pig. He found delayed absorption of the fracture hematoma, deficient production of osteoid trabeculae, and increased necrosis of the broken ends. Hertz also attempted to repeat Israel and Frankers refracture experiments, but with no success. Lexer obtained similar results, but he stressed the deficiency of blood supply as a cause of the failure of scorbutic tissues to heal. 

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