| Abstract: | Mammary explants from midpregnant mice were cultured for 4, 24, 48, and 72 hours in the presence and absence of insulin. Changes in the activities of phosphoglucose isomerase (an enzyme of the glycolytic pathway for glucose metabolism) and of glucose-6-phosphate and 6-phosphogluconate dehydrogenases (enzymes of the pentose phosphate pathway) were assessed at each culture interval. During the first four hours of culture, no significant effect could be attributed to insulin on the activity of these enzymes. Moreover, insulin had no detectable stimulatory effect on phophoglucose isomerase until 48 hours, at which time the hormone caused a marked increase in the activity of this enzyme over the next 24 hours. In contrast, insulin stimulated only a small increase in dehydrogenase activity at 24 hours, after which this hormone acted mainly to maintain the activity that was present initially. These results indicate differential actions of insulin on two groups of enzymes catalyzing the same substrate. |
