Multiple forms of glucose-adenosine triphosphate phosphotransferase in rat mammary gland

Measurements have been made of the total hexokinase activity and of the relative amounts of types I and II hexokinase in rat mammary gland and at different stages of the lactation cycle. The total hexokinase activity increased during lactation, that of type II increasing to a greater extent than that of type I; the type II/type I activity ratio rose from a pregnancy value of about 1 to a mid-lactation value of 3, returning to 1 on involution. The changes in type II hexokinase activity during the lactation cycle parallel the changes in the insulin sensitivity of mammary-gland tissue. A study of the effect of alloxan-diabetes on mammary-gland hexokinase during the mid-lactation period revealed that, although the total glucose-phosphorylating capacity of the mammary gland was almost unchanged, the relative contributions of type I and type II hexokinases altered, decreasing the type II/type I activity ratio to about 1.