3 The Enzymic Elimination of Ammonia

Abstract

Publisher Summary This chapter compares the four enzymes catalyzing the eliminations of ammonia, which include aspartate ammonia-lyase, 3-methylaspartate ammonia-lyase, histidine ammonia-lyase, and phenylalanine ammonia-lyase. The first cell-free preparation of aspartate ammonia-lyase was made from Bacillus fluorescens liquifaciens . Its presence has been demonstrated in many bacteria, fungi, a few higher plants, and in the livers of sharks, ducks, chickens, and frogs. The enzyme presumably allows microorganisms to use aspartate as a carbon source, however it may also have a synthetic function. At alkaline pH values the enzymes from Bacterium cadaveris and E. coli show sigmoidal kinetics. 3-Methylaspartate ammonia-lyase shows Michaelis-Menten type of kinetics at all pH values studied, although complications arise as it requires monovalent and divalent metal ions for activity. Evidence for the participation of an –SH group in the catalytic mechanism has been obtained from studies of the protection afforded by substrates and inhibitors against inactivation by sulfhydryl reagents. In mammals, the histidine ammonia-lyase is found in the skin as well as in the liver. The metabolic defect, histidinemia, has been described in children and has been shown to result from an absence of histidine ammonia-lyase activity. In bacteria, the enzyme participates in the catabolism of exogenous histidine. The enzyme has been extensively purified from Pseudomonas species in several laboratories. Phenylalanine ammonia-lyase is the only ammonia-lyase, which is widely distributed in plants. In higher plants, the enzyme diverts phenylalanine and tyrosine from protein synthesis to such phenylpropanoid compounds as lignin, flavonoids, conjugates of the hydroxycinnamic acids, and certain alkaloids. In microorganisms, the enzyme functions in the catabolism of exogenous amino acids, the cinnamic and p-coumaric acids formed being used as a carbon source.