1. Some of the kinetic properties of the particulate lactose synthetase of mouse mammary gland were investigated and shown to resemble those that have been reported for the soluble enzyme. Typical values for intact preparations were 2.3μm for the apparent Km for α-lactalbumin at 40mm-glucose and 1.7mm for the apparent Km for glucose at the endogenous concentration of α-lactalbumin. 2. Digitonin treatment increased total assayable activity approximately twofold but almost eliminated the endogenous activity found in the absence of α-lactalbumin, these findings being consistent with the location of endogenous activity within Golgi vesicles. 3. From the properties of the particulate fraction from lactating mice it was deduced that the effective endogenous α-lactalbumin concentrations were in the range 1–10μm. 4. The concentration of α-lactalbumin was not significantly different in particles isolated at various stages of pregnancy and early lactation. 5. The implications of these results for the control of lactose synthetase in vivo are discussed.
