SUMMARY: The histidine-induced production of α-toxin in strain Wood 46 declined rapidly when the pH was raised above 8·0 in the medium. This was related to the degradation of histidine by a histidase. The staphylococcal histidase was thermostable, had a of 1·2 × 10 and pH optimum near 9·0. Both histidine and urocanic acid raised the intracellular level of histidase when added to the medium. The enzyme was inhibited by cysteine, but not by EDTA. It was slowly oxidized to an inactive form, activity being restored by glutathione or mercaptoethanol.

Histidine dipeptides had a similar or more potent stimulating effect than histidine on the production of staphylococcal α-toxin in a synthetic medium. Glycyl--histidine and -alanyl--histidine entered the organisms more readily than free histidine. The dipeptides were hydrolysed intracellularly, and a considerable portion of the released histidine was further degraded to urocanic acid.

-Homocarnosine (γ-aminobutyryl--histidine) did not stimulate α-toxin production. The uptake and intracellular hydrolysis of this dipeptide were significantly slower than those of dipeptides with the free amino group in the α-position. Accumulation of intracellular histidine could not be demonstrated without an exogenous source of histidine. This was also examined under conditions when α-toxin was produced. The results indicated that stimulation of α-toxin production was not directly correlated with the intracellular level of free histidine. Accordingly, the mediator of the stimulating effect is still unknown.


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