Effect of Zn2+ on the proteolytic inhibitory action of insulin and biguanide antihyperglycemic drugs.
The involvement of Zn2+ in the inhibitory action of insulin and phenformin on bulk proteolysis was studied in the Langendorff rat heart with a Zn(2+)-buffering perfusate (0.1 mM citrate, physiological complete amino acids and 0.2% albumin). Proteins were biosynthetically labeled in vitro for 10 min with [3H]leucine. Rapidly degraded proteins were eliminated during a 3-h preliminary degradation without insulin or added Zn2+ (2 mM nonradioactive leucine). Insulin (5 nM), the lysosomal inhibitor chloroquine (30 microM), and the biguanide antihyperglycemic agent phenformin (2 microns) each caused a sustained 35-40% inhibition of [3H]leucine release beginning within 1-2 min and reaching a maximum at 1-1.5 h. When these agents were combined, their simultaneous proteolytic inhibitory effects were not appreciably greater than the effect of chloroquine alone. Infusion of supraphysiological perfusate Zn2+ (greater than 15 microM) mimicked the inhibitory effect of insulin and chloroquine on lysosomal proteolysis. Infusion of supraphysiological Co2+, Mn2+, Fe2+, and Cr3+ (30 microM, 0.5 h) caused no change in proteolysis; however, 30 microM Cu2+ caused a slight inhibition. Presumptive chelation of the background (approximately 20 nM) Zn2+ by infusion of 3 microM CaNa2 EDTA caused no change in protein degradation over 1-2 h. The infusion of a physiological concentration of 1 or 5 microM Zn2+ (as ZnCl2) caused no change in protein degradation over 1-2 h. Biguanides are known to reversibly form a Zn2+ complex with affinity less than that of Zn2+ for EDTA. Prior infusion of 3 microM CaNa2 EDTA inactivated the proteolytic inhibitory effect of maximal (2 microM) phenformin over at least 1.25 h of concurrent infusion.(ABSTRACT TRUNCATED AT 250 WORDS)
Thorne
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Department of Pharmacology, School of Medicine, Wright State University, Dayton, OH 45459.
Lockwood
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D000818:Animals; D002738:Chloroquine / Q000494:pharmacology; D002857:Chromium / Q000494:pharmacology; D003035:Cobalt / Q000494:pharmacology; D004492:Edetic Acid / Q000494:pharmacology; D006321:Heart / Q000187:drug effects*; D066298:In Vitro Techniques; D007328:Insulin / Q000494:pharmacology*; D007501:Iron / Q000494:pharmacology; D007700:Kinetics; D007930:Leucine / Q000378:metabolism; D008345:Manganese / Q000494:pharmacology; D009206:Myocardium / Q000378:metabolism*; D010629:Phenformin / Q000494:pharmacology*; D011480:Protease Inhibitors; D014176:Protein Biosynthesis; D011506:Proteins / Q000378:metabolism*; D051381:Rats; D015032:Zinc / Q000494:pharmacology*
D007328:Insulin; D011480:Protease Inhibitors; D011506:Proteins; D002857:Chromium; D003035:Cobalt; D008345:Manganese; D002738:Chloroquine; D004492:Edetic Acid; D010629:Phenformin; D007501:Iron; D007930:Leucine; D015032:Zinc