Enzymic activities in two populations of purified rat islet beta-cells

In terms of glucose sensing by pancreatic islet beta -cells, emphasis is cu rrently placed on both the role of glucokinase, with negligible activity of low-Km hexokinase(s), and the prevalence of the oxidative over non-oxidati ve modality of glycolysis, a situation tentatively attributed, in part at l east, to a low activity of lactate dehydrogenase. Conflicting information i s available, however, on the activity of both low-Km hexokinase(s) and lact ate dehydrogenase in purified beta -cell homogenates. This issue was reinve stigated, therefore, in two populations of purified rat islet beta -cells s elected on the basis of their low (betaL) or high (betaH) content in reduce d pyridine nucleotides. The size and protein content of betaH cells represe nted about twice that of betaL cells. Such was also the case for low-Km hex okinase(s), lactate dehydrogenase, mitochondrial FAD-linked glycerophosphat e dehydrogenase, glutamate dehydrogenase and glutamate-alanine and glutamat e-aspartate transaminases. Whether in betaH or betaL cells, the activity of low-Km hexokinase(s) was at least as high as or higher than that of glucok inase. In both betaH and betaL, the activity of lactate dehydrogenase excee ded that required to catalyze the full reduction of glucose-derived pyruvat e to L-lactate, as estimated from the rate of D-glucose phosphorylation und er physiological conditions. These findings thus argue against a low expres sion of either low-Km hexokinase(s) or lactate dehydrogenase as major deter minants of the glucose-sensing device in beta -cells.