1. 1. The properties of lactose synthetase from mouse mammary tissue were investigated and found to resemble those of the cow and rat. 2. 2. Lactose synthetase is composed of two subunits, A and B; both are required for lactose-synthetase activity. Gel filtration of partially purified homogenates of mouse mammary tissue on Sephadex G-100 reveals the molecular weight of B to be about 15 000. The A protein elutes as three peaks: one at the void volume, a second with proteins having a molecular weight of 100 000-130 000, and a third at 29 000. 3. 3. Crude homogenates of lactose synthetase have different properties than partially purified preparations. When crude preparations of lactose synthetase are diluted the resulting activity is proportional to the extent of dilution; however, the dilution of purified preparations results in an exponential decrease in activity. Also, restoration of lactose-synthetase activity by recombination of A and B subunits follows different kinetics with the two types of preparations. 4. 4. A third component, Z, has been proposed which in crude preparations has the properties of preventing rapid dissociation of the AB complex. During purification Z is lost and the A and B subunits then obey the laws of mass action and chemical equilibrium. © 1969.
