Regulation of two forms of glutamine synthetase of soybean hypocotyls

Abstract

The two forms of glutamine synthetase (GS-I, GS-II) isolated from etiolated soybean hypocotyls had similar molecular weights, but different elution properties from DEAE-cellulose, different apparent K m-values for glutamine and had different responses to inhibitors of GS. Alanine, the most inhibitory amino acid, resulted in a 35% inhibition at 1 mM (GS-I). Serine was a non-competitive inhibitor with respect to glutamine for GS-I. Increased phosphorylation of adenine nucleotides resulted in increased inhibition with ATP the most inhibitory (50% at 1 mM). CTP interacted with ATP synergistically and exhibited non-competitive inhibition with respect to glutamine for GS-II. Cumulative feedback inhibition was evident with amino acids and nucleotides for both enzymes. Pyrophosphate (5 mM) produced almost total inhibition of both activities with GS-II slightly more sensitive. These results indicate the involvement of amino acids and nucleotides in the feedback control of GS activity in soybean in a cumulative manner and the possible involvement of an energy charge system of regulation.