Protein synthesis in intestinal mucosa; the effect of route of administration of precursor amino acids

rat are capable of synthesizing protein from amino acid precursors (L-leucine). Moreover, polyribosomes from both crypts and villi are equally able to incorporate L-leucine into protein. Unlike other tissues, e.g. liver, there is no diurnal variation of protein synthesis in the intestine of the unfed rat, whether leucine is adminis-tered intraluminally or intravenously. The route of administration of precursor (L-leucine) is important in determining which part of the villus in-corporates the label into protein. After intravenous ad-ministration, protein from cells near the villus-crypt junction is most heavily labeled, whereas after intralu-minal administration protein from cells near the villus tip is most heavily labeled. The pattern of proteins most heavily labeled by radioactive precursor is different in the villus when compared with proteins from the crypt cells. Smaller molecular weight membrane-bound pro-teins are preferentially labeled in the crypt cells, whereas on the villus the pattern of labeling is more evenly dis-tributed among the various proteins. Moreover, intra-luminal leucine is utilized for protein synthesis to a greater extent than that in the blood, when the con-centration in both compartments is similar. Thus, intra-luminal and intravenous injections of labeled precursor are not equivalent. Both routes should be considered in data for experiments measuring intestinal protein synthesis