Detection of low molecular weight cysteine proteinase inhibitors by time-resolved fluoroimmunoassay

A time-resolved fluoroimmunoassay was developed for the detection of 3 human low molecular weight cysteine proteinase inhibitors, ACPI (cystatin A), NCPI (cystatin B), and gamma-trace (cystatin C). Polystyrene tubes or polystyrene microtitration strips were used as solid phase. The rabbit anti-inhibitor immunoglobulins were used as the capture antibody, and, when labelled with europium, also as the detector antibody. The threshold sensitivity of the tests was 0.1 ng/ml for NCPI and 1 ng/ml for the others. All the 3 cysteine proteinase inhibitors, ACPI, NCPI, and gamma-trace, were detected in pooled serum samples of patients with kidney failure. gamma-Trace seemed to be quantitatively the major and ACPI the minor inhibitor. No other low molecular mass cysteine proteinase inhibitor was detected after isoelectric focusing of the 12 kDa area of gel filtered human serum.