Ccxxi. the Error Due to Blood Arginase

THE possibility of an error in the determination of urea in blood by urease methods was brought to our notice by the fact that figures obtained by the direct estimation of urea in human blood corpuscles were obviously too high. This error appears to have been first reported by Behre [1923], who observed that when the soya bean urease method was applied to whole blood the urea obtained was partially determined by the concentration of urease extract employed, and that the substance responsible for this "extra urea" was chiefly contained in the corpuscles. She did not come to any conclusion as to the exact nature of this substance. Addis [1928], when estimating the urea in liver tissue with Jack bean urease, obtained a linear relation between the quantity of urea produced and the quantity of urease used. He considered that the extra urea was derived from the action of arginase in the liver on arginine in the Jack bean. He also found that the apparent concentration of urea in human blood might be varied by adding different amounts of urease. He does not state that the variation in the case of blood was due to arginase. Peters & Van Slyke [1932], in describing the urease method as applied directly to whole blood, state "...unnecessarily prolonged contact between blood and enzyme is to be avoided because of the possibility of slow formation of ammonia by the action of arginase in the blood cells on arginine in the enzyme preparation" and refer to the work of Addis and Behre which did not actually prove that the error was due to the action of blood arginase. This source of error in a standard method appeared to be of sufficient importance to justify a further investigation.