Chemically Reactive Sulfhydryl Groups of 1 -Aminocyclopropane-1 -carboxylate Deaminase

Abstract

1 -Aminocyclopropane-1 -carboxylate deaminase was inhibited by several sulfhydryl-modifying reagents. Inhibition by 5,5′-dithiobis(2-nitrobenzoic acid) were reversible. The chemically sensitive sulfhydryl groups were about 3 mol per mol of enzyme. A competitive inhibitor, l-serine, reduced modification of the sensitive sulfhydryl groups. The mechanism-based inhibition by β-chloro-D-alanine almost completely prevented the modification of sulfhydryl groups. The inhibition by monoiodoacetamide and N-ethylmaleimide was strengthened at pH values above 8.5, and in this range of pH, the maximum velocity of the enzyme reaction increased.

The enzyme had 2.8 mol of tightly bound pyridoxal 5′-phosphate per mol (110,000 g) and was dissociated to a single subunit of molecular weight 36,500 by sodium dodecyl sulfate. The purified enzyme showed pH-dependent absorption maxima at 326 and 416 nm. The absorbance at 416 nm decreased as the pH of the enzyme solution increased from 7 to 9. The decrease in the absorbance corresponded to the pH dependence of Km and Ki at pH values below 8.5. The maximal affinity of the enzyme to substrate and competitive inhibitors, l-alanine and l-serine, was at pH 8.5.