An Error in the Urease Method for the Determination of Urea. ∗

Abstract

Estimates of the urea concentration of the blood, muscle and liver tissue of rats were carried out in connection with a study of the effects of high protein diets. The urease method was used. A typical result is given in Table I.

The high concentration of urea shown here for the rat's liver is at variance with the observation of Marshall 1 who found an approximate equality of urea concentration in the blood, muscle and liver of other animals. In obtaining the results shown in Table I the rats were anesthetized with ether, and bled to death by cutting the abdominal aorta. The tissues were removed immediately, weighed, ground in a mortar and quantitatively transferred with water to large aeration tubes. The method used has been described in detail elsewhere. 2 The only changes made were those which were necessitated by the fact that the tissues and blood were made up with water to a volume of 30 cc. This method is simply a special adaptation of the aeration method described by Marshall. 1 The urea is calculated from the amount of additional ammonia formed after incubation of the tissue with the filtrate from a suspension in water of freshly ground jack beans.

When the liver was heatcd before adding the jack bean filtrate an entirely different result was obtained, the previous high level of urea concentration in the liver was no longer found, and the estimates showed an approximate equallity of concentration in blood, muscle, and liver, in accordance with the conclusion reached by Nf arshall. (Table 11.)

A linear relation is attained ktween the quantities of urea produced and the quantities of “urease” used when the ratio between liver and urease is neither too small nor too large. Fig. 1 shows the result of an experiment in which the liver was held constant at 2.26 gm. and the urease was varied from 0.2 to 1.0 gm. The mixtures were incubated for 2 hours at a temperature of 37” C. The reaction was maintained at pH 7.2 by phosphate.