The saturation with cofactor of the tryptophan pyrrolase of rat liver was markedly increased by intravenous administration of solutions of haematin. Complete saturation followed the administration of 0.5–1 μmole haematin per 100 g body weight or larger amounts. No significant increase in tryptophan pyrrolase activity accompanied or followed this apparent saturation of the enzyme with cofactor in vivo.
The dipeptides Asp-SER32P and Ser32P-Ala have been isolated from an acid hydrolysate of calf-intestinal alkaline phosphate (orthophosphoric monoester phosphohydrolase, EC 3.I.3.I), which had been 32P-labelled on a serine residue at the active site, as described earlier. This shows that the amino acid sequence aroun the active serine is ASP-Ser32P-Ala. The peptides were identified by using an acid hydrolysate of crytalline ovalbumin the same unballed peptides as reference substances.
1. The catalytic properties of the intracellular peptidase cathepsin C (EC 3.4.4.9) were investigated. The enzyme is devoid of the proteolytic activity that had been ascribed to it.
2. Cathepsin C has a very narrow specificity. Its activity is restricted to the removal of N-terminal dipeptide units that meet a number of rigorous requirements, from amides, esters or polypeptides.
3. In transamidation reactions cathepsin C shows a narrow specificity not only for the peptide “donor”—as in its hydrolytic activities—but also for the “acceptor” to which the dipeptide units are transferred.
4. These enzymic properties make a function of cathepsin C in intracellular protein catabolism seem improbable.
Among a number of microorganisms tested, only streptomycin- and hydroxystreptomycin-producing strains of Streptomyces contained high levels of the enzyme L-arginine: X amidinotransferase, so named because the physiological formamidine acceptor is not known. Of the several reactions common to known amidinotransferases, arginine: NH2OH transamidination was chosen for exploratory assays because of its unique compatibility with both single- and double-displacement reaction mechanisms. In high-enzyme strains, Streptomyces griseus ATCC 12475, Streptomyces bikiniensis ATCC 11062, and Streptomyces griseocarneus ATCC 12628, amidinotransferase activity is low from 0–24 h of growth on complex media, but increase 30-fold within the next 24 h. Lysozyme extracts of the derepressed mycelia have the highest amidinotransferase activity yet observed in nature. Mycelia harvested at 24 h and shaken in 1% NaCl exhibit a similar activity increase; this phenotypic change is prevented by low levels of neomycin. Since earlier workers have shown that streptomycin formation follows a similar time-course, the evidence is strong that X is an early precursor of the streptidine moiety of streptomycin. Biosynthesis of netropsin probably does not involve a transamidination.
Arginine: X amidinotransferase activity is strongly inhibited in vitro by cystine, cystamine, formamidine disulfide, and the mixed disulfide of cysteine and thiourea, but not by oxidized glutathione. These inhibitions are reversed by 2-mercaptoethanol.
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