The estimation of high concentrations of plasma acid phosphatase.

Plasma acid phosphatase estimation is of established value in the diagnosis of carcinoma of the prostate with bone secondaries, and in the assessment of stilboestrol treatment (Gutman and Gutman, 1938; Herbert, 1946; Wray, 1956). Modifications of the method of Gutman and Gutman (1938), using disodium phenyl phosphate substrate, and estimating the phenol liberated with Folin and Ciocalteu reagent, are widely used. When the result exceeds 40 units/100 ml. it is suggested that the estimation be repeated with diluted plasma (Varley, 1954). Herbert (1946) reduced the time of incubation as an alternative. Because of the low Michaelis constant for acid phosphatase, approximately 0.0006 (Sch0nheyder, 1952) and the relatively high concentration of substrate used in the standard technique, a linear relationship between enzyme concentration and activity would be expected over a wider range than the accepted range of 0 to 40 units/100 ml. The final blue complex produced by phosphotungstic /phosphomolybdic acids, phenol, and alkali, " goes to completion only in the presence of a gross excess of reagent and alkali" (Folin and Ciocalteu, 1927), so that dilution of the final coloured solution with water alone is obviously unsatisfactory. In the present investigation, dilution with reagent instead of with water extended this range of linearity.