STUDIES OF PROTEIN METABOLISM IN INTESTINAL EPITHELIAL CELLS*

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Abstract
The gastrointestinal mucosa contains several types of epithelial cells and other cellular elements. All cells synthesize protein, but the different types synthesize different kinds of protein at different rates. Hence amino acid incorporation and protein synthesis should be defined with reference to particular cell types. In this study leucine was used, labeled with tritium for microautoradiographic studies and with C14 for concomitant radiochemical analyses. Incorporation of leucine into rapidly proliferating epithelial cells located in the jejunal crypts was greater than in villus epithelial cells, shortly after injection of H3-leucine. Grain densities within the epithelial cells also varied. In villus epithelial cells nuclei were less heavily labeled than was cytoplasm, and cytoplasm adjacent to nuclei was less heavily labeled than cytoplasm adjacent to microvilli. Paneth cells showed heavy labeling over the granules after injection of H3-leucine, and rapid disappearance of label. The findings also revealed a high degree of stability of synthesized protein in rapidly proliferating crypt epithelial cells, and a slow loss of protein from mature cells during their migration to the villus tip. When mice were placed on a diet containing the same amount of calories but no protein, the colon and other tissues atrophied markedly, but the jejunum did not. In the jejunum the rate of cell proliferation was approximately normal. The rate of removal of leucine, however, was greatly reduced, indicating reutilization of endogenous amino acid or of other protein precursors derived from previously labeled protein.