An enzyme immunoassay of human polymorphonuclear leukocyte cathepsin G

An enzyme immunoassay has been developed for the quantitation of human polymorphonuclear leukocyte cathepsin G. The assay had a linear relationship over the range 0.23-4.7 nmol/l (6-125 ug/l) and a detection limit of 0.23 nmol/l (6 ug/l). Recovery in citrated plasma only occurred when the concentration was higher than 4.0 umol/l (110 mg/l). This effect could be overcome by diluting the plasma before adding the proteinase or by inactivating the proteinase before diluting it with plasma. The failure to detect cathepsin G in plasma was due to the plasma inhibitor alpha-2-macroglobulin masking the antigenic sites of the proteinase. Samples from several types of leukemia showed no detectable cathepsin G even when the total myeloid count was up to ten times the normal.