Regulation of enzyme turnover during tissue differentiation. Studies on 6-phosphogluconate dehydrogenase in rabbit mammary gland in organ culture.

(1) Explants of mammary gland from mid-pregnant rabbits were cultured in Medium 199 in the presence or absence of insulin, prolactin and cortisol. (2) Antiserum to 6-phosphogluconate dehydrogenase was raised in sheep and used to titrate the amount of enzyme activity present in explant extracts. Changes in enzyme activity were found to be due to corresponding changes in amount of the enzyme. The greatest increases in the amount of the enzyme were only brought about by culture of explants in the presence of hormones (insulin, prolactin and cortisol) in Medium 199 which contained glucose. (3) The increases in the amount of the enzyme were similar in explants cultured with hormones in Medium 199 which contained 1.39 mM, 5.55 mM or 55.5 mM glucose. (4) When explants were cultured with hormones in Medium 199 which contained glucose (5.55 mM) for 24 h and then cultured with hormones in Medium 199 which contained glycerol (10.9 mM), a decrease in the amount of the enzyme occurred. In contrast, the culture of explants with hormones in Medium 199 which contained glycerol (10.9 mM) for 24 h followed by transfer of the explants to medium which contained glucose (5.55 mM) resulted in an increase in the amount of the enzyme to reach values which were not different from those found in explants cultured throughout with hormones in Medium 199 which contained glucose.