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Biosynthesis of Lactose¹

Department of Biochemistry, Agricultural Experiment Station Oklahoma State University, Stillwater 74074

ABSTRACT

In 1952, Reithel et al. (29) observed the formation of lactose in homogenates prepared from lactating rat and guinea pig mammary tissue and it soon became apparent that a number of enzymes were involved in the conversion of glucose to lactose. Glucose is converted to glucose-6-phosphate by hexokinase (EC 2.7.1.1) and it is then isomerized by phosphoglucomutase (EC 2.7.5.1) to glucose-1-phosphate. Both of these enzymes are common to the glycolytic pathway. UDP-Glucose pyrophosphorylase catalyzes the formation of UDP-glucose which is then converted to UDP-galactose by the epimerase reaction. Both of these enzymes are found in many tissues including liver and brain. Gander et al. (22) suggested a scheme for the biosynthesis of lactose from UDP-galactose and glucose-1-phosphate to form lactose-1-phosphate which was hydrolyzed by a phosphatase to lactose. However, this scheme has not been verified and also studies with the incorporation of ¹⁴C precursors were not consistent with this pathway and these studies have been reviewed by Jones (24).

¹ Journal Article 2039 of the Agricultural Experiment Station, Oklahoma State University, Stillwater. This research was supported in part by grants from the American Cancer Society (P 420), National Institutes of Health (AM 10764), National Science Foundation (GB 7975), Agricultural Research Service, United States Department of Agriculture, Grant 12-14-100-9204 administered by the Eastern Utilization Research and Development Division, 600 East Mermaid Lane, Philadelphia, Pennsylvania, and a Career Development Award from the National Institute of Health, 1 KO4 GM 42396.