Benzpyrene which had been stored in the dark had no effect upon the activity of urease. The same benzpyrene which had been exposed in the solid state to ordinary laboratory illumination through a colorless glass bottle inhibited urease. This inhibitory activity disappeared when either the solid benzpyrene or solutions of benzpyrene were stored in the dark, and reappeared after the solid had been exposed to light again.

The activity of urease inhibitory benzpyrene was prevented by cysteine. This effect of cysteine, in conjunction with the results obtained after treatment of the urease samples with p-chloromercuribenzoate, indicates that the inhibitory effect of the benzpyrene was due to its action on the sulfhydryl groups of the urease.

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This investigation was supported by research grants from the National Cancer Institute of the National Institutes of Health, Public Health Service, and from the American Cancer Society.

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