Free amino acids in amniotic fluid and the prenatal diagnosis of homocystinuria with methylmalonic aciduria

CLINICAL CHEMISTRY, Vol. 41, No. 11, 1995 1663 strate having a composition optimal for ACE activity (8), account for the relatively high ACE unit values we report for this method. The presence of GGT does, however, accelerate an autolytic release of Gly-Gly from Hip-GlyGly in the reagent 1 solution, falsely increasing apparent ACE activity. Nonetheless, because calibrator activity and sample activity are affected equally, recalibration before each run fully corrects for this effect, even over a period of several days. This is shown by our between-run CV of 4.17% for replicate sampling over a 3-day period with the same calibrator sera. In the absence of Gly-Gly, GGT catalyzes the transfer of a y-glutamyl moiety of a GGCN (donor) molecule to a second (recipient) GGCN molecule. The reaction releases the 3-carboxy-4-nitroaniline chromophore from the GGCN donor molecule, just as it does when Gly-Gly is the acceptor substrate. Referred to as the autotransfer reaction (6), this therefore causes a significant t .A / t even at zero ACE activity (no released Gly-Gly). However, the contribution to z A/t by the autotransfer reaction is constant, irrespective of the presence of or concentration of Gly-Gly, the preferred acceptor substrate in the reaction. Reaction rates are therefore strictly linear with respect to Gly-Gly concentrations, and consequently to ACE activity (6).