Does leucine- and norleucine-induced insulin release depend on amino acid aminotransferase activity?

In the absence of another exogenous nutrient, L-leucine but not L-norleucine stimulates insulin release from rat pancreatic islets, although the corresponding keto acids, 2-ketoisocaproate and 2-ketocaproate, are equally potent secretagogues. Such a situation cannot be ascribed to the preferential transamination of L-leucine as compared to L-norleucine in islet homogenates. Indeed, in the presence of a suitable activator of glutamate dehydrogenase, L-leucine and L-norleucine stimulate secretion to the same extent. It is concluded that the rate of transamination of these amino acids in intact islet cells depends on the availability of a 2-keto acid partner rather than on the assayed amino acid aminotransferase activity.