The enzyme content of the brush border of the small intestine was measured after exposure of the abdomen of rats to 500 and 800 R, respectively, of gamma radiation from a cobalt-60 source. Enzyme activities determined between 4 hours and 16 days after exposure indicated that there were modifications in lactase, maltase, invertase, alkaline phosphatase, LAP (leucineaminopeptidase), and total protein content in homogenates of different segments of the small intestine. The first demonstrable effect, occurring between 4 and 36 hours after exposure was an increase in enzyme activity, which was more marked after 500 R than after 800 R. Thereafter the depression in enzyme levels was correlated with the degree of abnormality of the epithelium. Recovery was almost complete at 5 days after 500 R. Although the activity of some of the enzymes had partly recovered at that time after 800 R, full recovery to preexposure values had still not occurred at 11 days after exposure. Histologic damage to the epithelium, particularly in the villi, was more serious during the time of the greatest functional impairment.

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