Serum and milk alkaline phosphatase in human lactation

Serum and milk alkaline phosphatase (AP) values are reported for the first 16 weeks of lactation. Serum levels at all stages are elevates above normal-pregnant controls. Sequential milk AP values are reported for the first time. Heat inactivation and phenylalanine inhibition studies would suggest that serum AP in lactation is composed of bone and liver isoenzymes, but it shows an increase in the rate at which enzymatic activity is lost when frozen, indicating that lactation serum AP is largely a different isoenzyme. Electrophoresis demonstrates several AP "slow" bands of unknown origin in lactation serum.