A simple, solid-phase iodine-125 radioimmunoassay for theophylline.

The activity of y-glutamyltransferase (EC 2.3.2.2) is measured on the DuPont aca by an adaptation of the Szasz procedure ;-y-glutamyl-p-nitroanilide is transferred to glycylglycine, thereby releasing p-nitroaniline, which is measured at 405 nm. I have noticed low glutamyltransfer-ase activity in patients with cholestasis wherealkaline phosphate (EC 3.1.3.1) is increased, total bilirubin> 100 mg/L, and conjugated bilirubin > 75 mg/L. The manufacturer states (1) that icteric serum does not interfere with their methodology for glutamyltransferase. Combes et al. have stated(2) that conjugated bilirubin and unconjugated bilirubin will depress glutamyltrans-ferase activity significantly. In contradiction of these findings, Dickson and Beck (3), using bilirubin from human bile and crystalline bilirubin, stated that there was no apparent inhibition except at bilirubin concentrations of 140 and 280 mgfL (bilirubin from bile). Their conclusion was that other, unidentified factor(s) contribute to this phenomenon. Theodorsen and Stromme (4) added bilirubin (source not stated) up to 200 mg/L to serum but found no measurable inhibition of glutamyltransferase activity. They state that the change in absorbance when assaying for glutam-yltransferase may be nonlinear in a sample with high background absorb-ance. Bilirubin alone or in combination with other components of this assay would give a high background absorb-ance at 400-410 nm. In addition, they report that glutamyltransferase inhibition by donor substrate and acceptor varies with the instrument and methodology. Analysis of a serum with total biliru-bin at 150 mg/L, conjugated bilirubin at 75 mg/L, and glutamyltransferase activity of 44 UfL gave results of 80 and 100 units of glut.aniyltransferase activity per liter when diluted 10-and 20-fold with an albumin-based diluent. However , assay of several samples with high glutamyltransferase activity diluted with equal volumes of sera having conjugated bilirubin > 75 mg/L and low glutamyltransferase levels showed no inhibition by bilirubin (result always halfway between respective glutamyl-transferase levels). The problem may be due to direct enzymatic interference by conjugated or unconjugated bilirubin (or both) or by other unidentified compound(s), to instrumental error from increased background absorbance at 405 nm, or to interference with substrate-enzyme interaction by the inhibitor. The manufacturer states that icteric serum does not interfere with their methodology; however, in their studies they used crystalline bilirubin, so inhibition by conjugated bilirubin would not be observed. evaluation of a y-glutaniyl transferase method for the DuPont aca. al., Serum 'y-glutamyl transpeptidase activity in viral hepatitis: Suppression in pregnancy and by birth control pills.tamyltransferase activity and bilirubin concentration. Radioimmunoassay (RIA) offers …