Abstract
Administration of radioactively labeled galactose to cultured mammalian cells brings about an accumulation of metabolic products the pattern of which seems to be governed by a variety of vectors in the intracellular milieu. By manipulation of culture conditions some of these vectors appear to be a function of glycolysis. In the non‐glycolytic culture, label from a galactose probe appears as Galactose‐1‐phosphate (Gal‐1‐P) and UDPglucuronic acid (UDPGlcUA). Conversely, glycolytic culture conditions seem not to permit the formation to UDPGlcUA since the only labeled accumulation product formed was UDPHex. A suggestion is made that the difference in metabolic activity of glucose‐fed and glucose‐starved cultures may be related to the effect of NADH on the in situ activity of UDPG dehydrogenase (UDPglucose:NAD oxidoreductase, E.C. 1.1.1.22) (abbreviation, UDPG‐DH). This prompted an investigation of the effects of NAD and NADH on the activity of partially purified UDPG‐DH. The results of these experiments strongly suggest that the activity of UDPG‐DH (in situ) is negatively controlled by increased levels of NADH; the latter condition is known to exist in glycolytically active cells (Schwartz and Johnson, 1976). Added to this is a second control mechanism which is characterized by a transient inhibition of uridylyltransferase (UDP glucose:α‐D‐galactose‐1‐phosphate uridylyltransferase, E.C. 2.7.7.12). Since it is known that there is little, if any, effect on galactokinase (ATP:D‐galactose‐1‐phosphotransferase, E.C. 2.7.1.6) activity as a result of sugar starvation (Christopher et al., 1976), the low in vivo activity of uridylyltransferase contributes not only to the increased accumulation of Gal‐1‐P but also to a drastic decrease of labeled UDPhexoses, although the pre‐existing pool of UDPhexose was found to decrease only moderately under the condition of glucose starvation (30% still persisted). The benefit of this type of control is not clear.