A number of peptides have been synthesized which represent portions of the tetradecapeptide renin substrate molecule, and which contain the hydrolyzable leu-leu bond. An automatic chemical method for determination of the velocity of the reaction of renin with these compounds was developed. Application of the method at several levels of substrate concentration permitted construction of Lineweaver-Burk plots, and calculation of Michaelis constants (Km) and maximal velocities (Vmax). The results show that the maximum affinity of the enzyme (lowest Km) for substrate is achieved only with the full tetradecapeptide molecule (asp1-arg2-val3-tyr4-ileu5-his6-pro7-phe8-his9-leu10-leu11-val12-tyr13-ser14). Removal of asp1 and arg2 from the N-terminal increases the Km eight-fold. Further, moderate increase in Km occurs when the next amino acids, val3, tyr4 and ileu5, are removed. The further removal of his6 results in a marked reduction in the Vmax. Removal of ser14 from the C-terminal of the nonapeptide his6-pro7-phe8-his9-leu10-leu11-val12-tyr13-ser14 does not greatly affect the Km nor the Vmax. Further removal of tyr13 from this compound results in complete loss of substrate activity. It is suggested that the compounds his6-pro7-phe8-his9-leu10-leu11-val12-tyr13-ser14 or his6-pro7-phe8-his9-leu10-leu11-val12-tyr13 might be used as substrates for the chemical assay and standardization of renin.

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