Ribonucleic acid metabolism in the liver after administration of individual amino acids.

The effects of protein deficiency on the metabolism of ribonucleic acid in the liver have already been described (Munro, Naismith & Wikramanayake, 1953; Clark, Naismith & Munro, 1957). The amount of ribonucleic acid in the liver falls rapidly during the first day of protein depletion and then becomes adjusted at a lower level. During the initial phase of reduction, uptake of 32p and of [14C]glycine by ribonucleic acid is greatly diminished. This is probably due to breakdown products of ribonucleic acid causing dilution of labelling among the precursors used for its synthesis. Feeding of protein prevents this dilution effect, as is evidenced by the rapid restoration of labelling of ribonucleic acid. This suggests that the breakdown is partly or wholly suspended while dietary amino acids are being absorbed from the gut. These findings led us to conclude that the stability of liver ribonucleic acid is determined by the supply of amino acids for synthesis of protein. Consequently, it was of interest to know whether the metabolism of liver ribonucleic acid is affected by feeding with a protein deficient in one or more essential amino acids. This question has now been examined by using zein as the test protein. Contrary to expectation, zein was found to be almost as effective as casein in obliterating evidence of breakdown of ribonucleic acid. Extension of these studies to mixtures of amino acids confirmed that changes in metabolism of ribonucleic acid are not dependent on all essential amino acids being fed simultaneously. Individual amino acids were then administered singly, and it was found that glycine, methionine and leucine were each capable of producing a considerable effect on the metabolism of liver ribonucleic acid. The high leucine content of zein is sufficient to account for a large part of its action on the metabolism of ribonucleic acid. EXPERIMENTAL METHODS