The synthesis of neuramin lactose by preparations of rat mammary gland and its relation to the synthesis of lactose.

Lactose and neuramin lactose were produced by incubating subcellular particles from rat mammary gland in a system containing glucose, uridine diphosphate galactose, and cytidine monophosphate N-acetylneuraminic acid. The K, values for glucose in the synthesis of neuramin lactose were fairly constant for three similarly prepared enzyme preparations (0.25, 0.17, and 0.22 m&r), but were widely dispersed in the synthesis of lactose (0.93, 1.8, and 18 II~M). For UDP-galactose the K,,, value in the synthesis of neuramin lactose was 0.01 mu; in the synthesis of lactose it was 0.15 mnx. The above mentioned system for the synthesis of neuramin lactose was compared with another in which lactose substituted for UDP-galactose plus glucose. The K, value for lactose was 5.6 m&r. The K, value for CMP-ZVacetylneuraminic acid in the system containing UDP-galactose plus glucose was 0.066 mu. The rate of production of neuramin lactose in the system containing lactose was lower than in the system containing UDP-galactose plus glucose. In the latter system an intermediate particle-lactose complex was isolated by centrifugation; on incubation of this complex with CMP-ZV-acetylneuraminic acid, neuramin lactose was produced. The formation of this intermediate complex is probably common for the synthesis of lactose and neuramin lactose; on addition of CMP-N-acetylneuraminic acid to a system synthesizing lactose, neuramin lactose was produced while corresponding amounts of lactose disappeared. No indication was obtained that the enzyme system which produces neuramin lactose from UDP-galactose plus glucose could be solubilized. No success was achieved in attempts to separate the particles which synthesize lactose from those which synthesize neuramin lactose from UDPgalactose plus glucose. The neuramin lactose produced from lactose and that produced from UDP-galactose plus glucose were cbromatographically identical.