[Presence of histidine-degrading enzymes in mycobacteria and nocardia and reaction-kinetic studies on Mycobacterium smegmatis SN 2 (author's transl)].

The occurrence of histidine degrading enzymes in whole cells of mycobacteria and nocardia was investigated. Out of 25 Mycobacterium strains, only M. smegmatis showed "histidase" activity, i.e. an uptake of histidine and a simultaneous release of ammonia. Consequently, the presence of "histidase" is a characteristic feature for M. smegmatis. In contrast to "histidase", single mycobacteria strains were able to take up histidine from the reaction mixture without liberation of ammonia or detectable oxidation of the substrate. This property was not species-specific. Therefore the determination of the released ammonia is decisive for the determination of the species-specific "histidase". Strains of the genus Nocardia proved to be more active concerning "histidase". Out of 18 species we found activity in all 4 N. erythropolis strains and in 7 out of 10 N. asteroides strains; the only strains of the species N. restrictus, N. opaca, N. blackwellii, N. paraffinae and R. terrae also showed "histidase" activity. Under the experimental conditions whereby 1 mmol/l of histidine and 10 mg/ml of bacteria were used, histidine was degraded by M. smegmatis within 6--8 hours; during which reaction 1,7--1,8 mmole/l of ammonia was released and 5/2--6/2 mmol of oxygen consumed. Other metabolites, such as urocanic acid or glutamic acid were not released in the reaction mixture. The kinetics of the degradation by whole cells was investigated in detail; the Michaelis constant was 0,53 . 10(-3)M, the optimal temperature found at 43,2 degree C, below and above which temperature the activation energies were 5177 and 11806 cal, respectively. "Histidase" is inducible in M. smegmatis. After cultivation of the bacteria on media containing histidine or urocanic acid, the enzymatic activity strongly increased. The same effect could be obtained by preincubating washed cells in buffer containing histidine or urocanic acif for 4 hours. "Histidase" was inhibited by streptomycin, viomycin and p-chlormercuribenzoate. The degeneration pathway is supposed to start from histidine via urocanic acid and glutamic acid; thus the proposed pathway is different from that one suggested by Bönicke (1964). Details of the degradation pathway using partially purified enzyme preparations will be described latter.