Activity Patterns of Several Enzymes of Liver,Adipose Tissue,and Mammary Gland of Virgin,Pregnant,and Lactating Mice. ∗

Abstract

Summary

Glucose-ATP phosphotransferases (glucokinase and hexokinase), glucose-6-PO₄ dehydrogenase, malic enzyme, and citrate-cleavage enzyme were assayed in the 100,000 × g supernatant fraction (Fraction III) of liver, inguinal fat pads cleared of mammary gland tissue, abdominal adipose tissue, and mammary gland excised from virgin, pregnant, and lactating mice of the C₃H strain. In addition, the activity of glucose-ATP phosphotransferases in the 350 × g supernatant fraction (Fraction I) and that associated with mitochondria of these same tissues was measured. Mitochondria isolated from adipose tissue contained only a small quantity of glucose-ATP phosphotransferase activity. No evidence for such activity was detected in mitochondria isolated from livers of lactating mice. Mitochondria from the mammary glands of lactating mice contributed more than 60% of the total glucose-ATP phosphotransferase activity of the cell, whereas those from virgin and pregnant mice contributed less than 17%.

The specific activities of malic enzyme and citrate-cleavage enzyme were higher in mammary glands of virgin and lactating mice than in those of pregnant mice. Glucose-6-PO₄ dehydrogenase activity was higher in liver and mammary gland of lactating mice than in virgin and pregnant mice. In adipose tissue, glucose-6-PO₄ dehydrogenase was lower in lactating mice than in virgin and pregnant mice. The effect of lactation on the activity of the enzymes of adipose tissue and liver indicates that lactation involves more than just the mammary gland. The response cannot be brought about in all tissues by a generalized mechanism since the pattern of the enzymatic change was not the same in all tissues studied. The finding that the intracellular distribution of glucose-ATP phosphotransferase activity in liver and adipose tissue was unchanged during lactation adds strong support to the view that the increase in hexo-kinase activity associated with mitochondria of lactating mammary glands is due to an increase in the proportion of parenchymal tissue in the gland.