Studies on the Site of Interaction for Ninhydrin, Alloxan, and D-Glucose in Insulin Secretion by Isolated Islets in Vitro*

Ninhydrin (85 /XM), a stable analog of alloxan, inhibited insulin release in a time-dependent manner. When Dglucose was present immediately after brief exposures of 1-2 min, part of the subsequent inhibition by ninhydrin was prevented. Ninhydrin inhibition of glucose-induced insulin release was not mediated by inhibition of the facilitated transport of Dglucose. These findings are identical to the in vitro interactions of alloxan with the insulin release mechanism and suggest that alloxan and ninhydrin inhibit secretion by a similar mechanism. The uptake of [3H]ninhydrin was measured in isolated islets and shown to be rapidly accumulated. The common interaction between alloxan and ninhydrin was further supported by the ability of alloxan pretreatment to alter the subsequent islet content of [3H]ninhydrin. D-Glucose decreased the islet content of label derived from ['H]ninhydrin under conditions which prevented ninhydrin inhibition of insulin release. The testagents, 3-O-methyl-D-glucose, caffeine, and L-g ucose, were ineffective both in preventing ninhydrin inhibition of insulin release and in altering the uptake of [3H]ninhydrin. The efflux pattern of label derived from the islets exposed to [3H]ninhydrin (85 JUM) for 5 min was rapid, with a majority (82%) of the radioactivity lost from the islets. The amount of undissociated abel retained by the islets, however, was decreased if the 5-min exposure period with [3H]ninhydrin (85 JUM) was performed in he presence of D-glucose These results suggest that a glucose-sensitive portion of the islet content of [3H]ninhydrin was related to ninhydrin inhibition of insulin release and that [:)H]ninhydrin may serve as a radioactive ligand to identify the cellular site in common with alloxan and D-glucose