Comparative properties of genetically defined peptidases in maize

Abstract

Four aminopeptidase isozymes (AMP1-AMP4) and an endopeptidase (ENP) from maize were purified by ammonium sulfate fractionation, DEAE-Sephadex ion-exchange chromatography, and Sephadex G-150 gel filtration. Hydroxylapatite chromatography further purified some of the peptidases. Comparisons of MW, substrate specificities and responses of peptidases to various reagents were made. The aminopeptidases varied in reactivities with the naphthylamide derivatives of amino acids. AMP1 and AMP3 were most active with the arginine and lysine derivatives; AMP2 was most active with the alanine and glycine derivatives and AMP4 was most active with the phenylalanine, tyrosine, leucine and tryptophan derivatives. MW as determined by gel filtration on Sephadex G-150 were 92,000, 86,500, 83,000, 61,000 and 67,600 for AMP1, AMP2, AMP3, AMP4, and ENP1, respectively. AMP2 had a MW of 88,000 as estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. AMP2 hydrolyzed the dipeptide derivatives, glycylglycyl-.beta.-naphthylamide and glycylphenylalanyl-.beta.-naphthylamide. Aminopeptidases were strongly inhibited by Zn2+, Cu2+, Hg2+, and p-mercuribenzoate. AMP1, AMP2, and AMP3 were inhibited by 1,10-phenanthroline, whereas AMP4 was not. AMP4 closely resembled aminopeptidases purified from barley grains and pea seeds. ENP was inhibited by p-mercuribenzoate and tosyllysine chloromethyl ketone.