1. Glutathione reductase and glutathione–cystine transhydrogenase activity in supernatant fractions of whole homogenates and homogenates of mucosal and muscular layers were determined in developing rat intestine after determination of the optimum conditions for assay of the two enzymes. In jejunum from adult rat, the Km values for GSSG reductase and GSH–cystine transhydrogenase activities were 0.25mm-GSSG and 0.23mm-cystine respectively. 2. The two activities could be differentiated by stability studies since GSSG reductase was stable at 60°C for 10min and could be stored at 4°C for 24h without loss of activity. GSH–cystine transhydrogenase, on the other hand, was denatured at 60°C and completely inactive after 24h storage at 4°C. 3. Based on calculations of total activities, both enzymes increased from the eighteenth day until the animals were young adults. 4. Total GSSG reductase activity increased at a greater rate with age than total GSH–cystine transhydrogenase activity as evidenced by activity ratios for GSH–cystine transhydrogenase/GSSG reductase of 0.44 and 0.12 in ileum from suckling and adult rats respectively, and 0.31 and 0.24 in jejunum from suckling and adult rats respectively. 5. In mucosa from adult rats GSSG reductase was more active in the ileum than in the jejunum, whereas GSH–cystine transhydrogenase activity was higher in the jejunum. 6. GSH–cystine transhydrogenase was active only in the muscle cells of the ileum of 7-day-old rats but became localized primarily in the mucosal layer in the adult rat. However, GSSG reductase activity was distributed evenly between the two layers throughout the intestine.

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