Changes in the enzyme pattern of the mammary gland of the lactating rat after hypophysectomy and weaning.

1. The enzymes glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, phosphoglucomutase, UDP-glucose pyrophosphorylase, phosphofructokinase, ATP-citrate lyase and acetyl-CoA carboxylase have been assayed in rat mammary glands in various stages of involution after hypophysectomy and weaning. 2. After hypophysectomy all seven enzymes decline in activity over a 12-16hr. period but the extent of the decline varies, with acetyl-CoA carboxylase becoming almost totally inactive, ATP-citrate lyase and phosphofructokinase showing a large decrease, and the remaining enzymes a less marked decline. 3. Within 24hr. of removing the litter a change in the pattern of enzyme activity is found very similar to that after hypophysectomy. 4. The significance of these results is discussed in relation to the endocrine control of mammary gland metabolism and the mechanisms of involution.