Cathepsin B/cystatin C complex levels in sera from patients with lung and colorectal cancer

A sandwich-type ELISA has been developed for quantification of the complex between the cysteine proteinase cathepsin B (CB) and its reversible tight-b inding inhibitor cystatin C (CC) in normal and pathological sera. The assay is based on a combination of catching Ab (3E1), raised against CB, and a h orseradish peroxidase-labelled detection Ab (1A2), raised against CC. Only the CB/CC complex is able to evoke a signal in this assay. The detection li mit of the assay was 15.5 nM and the working range between 31.3-200 nM. The within and between-run coefficients of variance (CV) varied from 4.7% to 9 .4% and 11% to 12.8%, respectively, demonstrating satisfactory reproducibil ity of the method. The concentration of the CB/CC complex was determined in sera from 90 healthy controls, 32 patients with non-cancerous lung disease s, 148 patients with lung and 32 patients with colorectal cancer. The CB/CC complex was significantly less abundant in sera of patients bearing malign ant lung tumours than in those with non-cancerous lung diseases or healthy controls (p <0.001). In colorectal cancer sera its level was significantly lower in advanced stages C and D than in early Dukes' stages A and B (p=0.0 2). Our results show that the increased levels of CB in malignant sera are not impaired effectively by CC and support the hypothesis of hindered inhib itory capability during cancer progression.