An enzyme which catalyzes the coversion of arginyl residues to citrullyl residues in protein was obtained from the extract of the epidermis of newborn rats. The enzyme required Ca2+ for its activity. The enzyme activity was enhanced in the presence of DTT. The maximum activity was observed at pH 7.5 at 50 degrees C in the presence of 10 mm CaCl2 and 2 mM DTT. The activity was inhibited strongly by treatment of the enzyme with monoiodoacetate or PCMB, which suggests that the epidermal enzyme is an SH-enzyme. The molecular weight of the enzyme was calculated by gel filtration to be about 48,000. It was essential for the alpha-amino or alpha-carboxyl group of the L-arginine substrate to be involved in a peptide linkage. The enzyme showed marked activities towards N-substituted L-arginine derivatives such as BZ-L-Arg, BZ-L-Arg-NH2, and BZ-Gly-L-Arg, But the action of the enzyme on free L-arginine was negligible. The enzyme activity was affected by the nature of the residue neighboring the arginyl residue in proteins. The authors propose the name "peptidylarginine deiminase" for this enzyme. A considerable specificity of the enzyme for proteins from the epidermal cells in terminal differentiation was observed. The results suggest that citrullyl residues in membranous protein of horny cells of the epidermis of newborn rat are formed by the action of epidermal peptidylarginine deiminase.

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Vitamin-enriched, lyophilized serum (VES) was prepared for an inter-laboratory study to compare vitamin assays. The VES contained water-soluble vitamins (vitamin B1, vitamin B12, vitamin C, and folate), fat-soluble vitamins (vitamin A and vitamin E), and cholesterol. We performed stability studies and determined vitamin concentrations and total cholesterol in VES stored at -20 degrees C for 12 months. Our recovery of the water-soluble vitamins in reconstituted VES was 70-142%, but we recovered only 33-45% of the fat-soluble vitamins. Physicochemical properties, such as specific gravity and viscosity of the reconstituted VES did not affect manual or automated measurements of these vitamins. Vial-to-vial differences found for the VES were the same as the within-day analytical variations. There was no evidence of degradation of vitamin A, vitamin B1, vitamin B12, vitamin C, folate, and cholesterol over 12 months in VES stored at -20 degrees C. Following deproteinization, vitamin C concentration was found to be lower than when not deproteinated. Vitamin E was less stable in VES, however, and the degradation during 12 months was lower than the between-day analytical variation of the assay. Our VES is the first preparation of lyophilized control serum that contains water-soluble and fat-soluble vitamins.

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