formycin and formycin-triphosphate

Formycin A augments insulin release evoked by glucose (5.6 mm or more), this effect not being rapidly reversible. The mechanism responsible for the insulinotropic action of formycin A was investigated in isolated pancreatic islets. It could not be ascribed to facilitation of glucose metabolism. On the contrary, formycin A inhibited glucose oxidation, lowered ATP content, and impaired glucose-stimulated protein biosynthesis. The insulinotropic action of formycin A was apparently attributable to its conversion to formycin A 5'-triphosphate, both this process and the secretory response to formycin A being abolished by the inhibitor of adenosine kinase 5-iodotubercidin. In agreement with the latter view, adenosine receptor antagonists such as 8-cyclopentyl-1, 3-dipropylxanthine and 3,7-dimethyl-1-propargylxanthine failed to suppress and, instead, augmented the insulinotropic action of formycin A. Unexpectedly, however, formycin A failed to decrease 86Rb efflux, this coinciding with a low efficiency of formycin A 5'-triphosphate to inhibit KATP-channel activity in excised membranes and with the fact that formycin A increased gliben-clamide-stimulated insulin release. The secretory response to formycin A represented a Ca2+-dependent process suppressed in the absence of extracellular Ca2+ or presence of verapamil and associated with an increased net uptake of 45Ca. Nevertheless, the view that formycin A exerts any major effect upon intracellular Ca2+ redistribution, protein kinase C activity, or cyclic AMP net production also met with objections such as the minor secretory effect of formycin A in islets exposed to a high concentration of K+ in the presence of a diazoxide analog, the resistance of formycin A insulinotropic action to bisindolylmaleimide, the poor increase of cyclic AMP content in formycin A-stimulated islets, and the pronounced enhancement by forskolin or theophylline of insulin release from islets exposed to formycin A. It is concluded, therefore, that the mechanism of action of formycin A in the pancreatic beta-cell remains to be elucidated.

Topics: Adenosine; Animals; Calcium; Diabetes Mellitus, Type 2; Diazoxide; Female; Formycins; Glucose; Glyburide; In Vitro Techniques; Insulin; Insulin Secretion; Islets of Langerhans; Rats; Ribonucleotides; Tubercidin; Verapamil; Xanthines

Protein kinase was isolated from both amastigotes and promastigotes of Leishmania mexicana amazonensis. Unlike the previously described enzyme from L. donovani promastigotes, activity of the L. mexicana kinases was 2-3 times higher at low ionic strength than at high ionic strength, and was 3-10-fold augmented by removal of endogenous low molecular weight inhibitors. The Km of the L. mexicana kinases was 123-223 microM, compared to the value of 70 microM for the beef heart kinase. Purine nucleoside analogs are potent antileishmanial agents that are phosphorylated to their respective triphosphates. The mechanism of the analogs is thought to involve competition of the triphosphates with ATP. Cordycepin triphosphate inhibited the amastigote, promastigote, and beef heart protein kinases approximately equally. However, the Kis of formycin A triphosphate for the leishmanial kinases (Ki 40-120 microM) were far less than that of the beef heart kinase (Ki 1,380 microM). The mechanisms of certain chemotherapeutic purine nucleosides may involve specific inhibition of leishmanial protein kinase by the nucleoside triphosphate.

Topics: Adenosine Triphosphate; Animals; Antimony Sodium Gluconate; Antiprotozoal Agents; Deoxyadenosines; Formycins; Leishmania mexicana; Pentamidine; Protein Kinase Inhibitors; Protein Kinases; Ribonucleotides; Suramin