Carbohydrate catabolism and the enhancement of uptake of galactose in hamster cells transformed by polyoma virus

Abstract

Untransformed as well as polyoma virus-transformed hamster cells can be grown equally well on a slow catabolite like galactose as on a rapid catabolite like glucose. The rate of uptake of galactose is greatly enhanced in the transformed cells as compared with untransformed cells, and this enhancement of entry was as markedly expressed in galactose-grown cultures as in glucose-grown cultures. Since the transformed cultures grown in glucose medium consume practically all of their carbohydrate, contrary to the galactose-grown cultures, problems dealing with regulation of transport by substrate concentrations have to be dealt with also. The galactose captured by the cells accumulates initially as galactose, alpha-galactose-1-phosphate, and UDP-galactose. However, after a 24-hr growth on a galactose growth medium, the product accumulated was almost exclusively galactitol. In spite of the enhancement of entry of galactose into the transformed cells, the metabolic pathway becomes stalled even before it has reached the stage of glucose-1-phosphate, largely due to a choke of the enzyme UDP-galactose-4-epimerase (EC 5.1.3.2). Among the sparse amounts of catabolic products generated by the transformed cells from galactose, carbon dioxide (allebeit no D-xylose) and lactic acid were found, both of them in much smaller amounts than seen if glucose is the carbohydrate source. Also, growth of transformed cells on a galactose medium gradually tends to become oriented, a phenomenon that could be called "contact promotion." Subsequent addition of glucose disturbs the oriented growth and interferes with contact promotion.