Plasma protein turnover and transmission to the milk in the rat

Abstract

1. 1. The protein composition of rat serum and whey was studied by paper electrophoresis in non-lactating rats and rats on the 13th and 20th days of lactation, and the turnover of serum proteins and their transfer to the milk were measured using 131I-labelled purified rat albumin, acidic α 1- glycoprotein , haptoglobin, transferrin and γ-globulin. 2. 2. Lactation was accompanied by a decrease in serum albumin concentration and by increases of the α- and β-globulins. Whey contained lower concentrations of the electrophoretic fractions, but showed a marked increase in β-globulin between the 13th and 20th days of lactation. The fractional and absolute turnover rates of all five proteins were greater in the lactating than in the non-lactating rats, and all of the proteins were transferred to the milk. It is concluded that the rates of catabolism of all the proteins were increased during lactation due to loss into the milk, and that there was also an increase in their rates of synthesis. With the exception of transferrin the relative rates of transmission of the proteins to whey was in inverse relation to their molecular weight, suggesting that the mammary gland acts as a molecular sieve for serum proteins.