In this report we demonstrate how the recently developed biotinylated affinity label biotinyl-Phe-Ala-diazomethane (Bio-Phe-Ala-CHN2) [Cullen, McGinty, Walker, Nelson, Halliday, Bailie & Kay (1990) Biochem. Soc. Trans. 18, 315-316; Walker, Cullen, Kay, Halliday, McGinty & Nelson (1992) Biochem. J. 283, 449-453] can be used for the detection of a precursor form of a cathepsin B-like enzyme produced by breast-tumour cells in culture. Thus the cell lines MDA-MB-436, ZR-75-1 and T47-D produce a soluble protein that can be allowed to react with the biotinylated affinity label to yield an SDS-resistant complex; this can be revealed with a streptavidin/alkaline phosphatase label after PAGE and Western blotting. This protein (molecular mass 47 kDa) can also be detected by immunoblotting using sheep anti-(cathepsin B) antibodies in conjunction with a donkey anti-sheep IgG label. None of the cell lines studied produced any mature cathepsin B-like activity, as gauged by the lack of turnover of the fluorogenic substrate benzyloxycarbonyl-Arg-Arg-4-methylcoumarin-7-ylamide (Cbz-Arg-Arg-NH-Mec). However, treatment of medium samples with pepsin resulted in the generation of such activity. When the pepsin-catalysed activation step was analysed by SDS/PAGE, the protein of 47 kDa was completely converted into two species of very similar molecular masses of 30.5 kDa and 29 kDa. Both these proteins can incorporate the biotinylated probe and, in common with the 47 kD species, they can be detected with the streptavidin/alkaline phosphatase label and immunoblotting. We propose that the 47 kD form is the pepsin-activable proform of these lower-molecular-mass species. The release of the proform from the oestrogen-receptor (ER)-positive breast-tumour cell lines ZR-75-1 and T47-D is stimulated 5-10-fold when these cells are grown in medium containing epidermal growth factor (EGF) at a concentration of 10 ng/ml. In contrast, there is no modulation in the amount of proform released by the ER-negative cell line MDA-MB-436, over a range of EGF concentrations from 0 to 100 ng/ml.

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