The proteinase present in acetone-dried extracts and in Fruton-type extracts of fresh skin from the rat have been compared and further characterized in the light of Fruton's results.2. The acetone-dried extracts have high pro-teinase activityand low dermo-and amino-peptidase activity with only slight activation effects of manganese, whereas the fresh skin extracts have a high peptidase and low proteinase activity.3. The skin proteinase differs from,previously recognized enzymes.4. Manganese activates the leucine aminopeptidase optimally at 0-002M; cobalt and magnesium also activate.We are indebted to Prof. K. Linderstrom-Lang for his hospitality and kindness in showing his technique; also to the Nuffield Committee and to the Osler Trustees (Oxford)

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I952 the amount of urocanic acid produced it will be impossible to assess what fraction of the histidine is metabolized by the route involving urocanic acid. The hypothetical system, shown on the previous page, proposed by Edlbacher would, however, appear to have more foundation in fact than he thought, since both enzymes A and B as well as C have now been shown to exist side by side in liver. SUMMARY 1. Methods for the identification of urocanic acid (,-4(5)-glyoxalinylacrylic acid) based on its ultraviolet absorption spectrum and on its separation from other iminazole derivatives by partition chromatography are described. 2. No urocanic acid has been observed in fresh liver homogenates or extracts from acetone-dried powders obtained from aqueous extracts of liver. 3. An enzyme capable of producing urocanic acid from histidine has, however, been shown to be present in such aqueous extracts. 4. The name histidine a-deaminase is suggested for this enzyme. 5. The enzyme has an optimum pH in the region of 7-8. 6. It appears to be completely inhibited by the product of its reaction.