A low-affinity binder for methotrexate in human leukaemia cells.

T h e commonly used trypsin substrate Na-benzoyl-m.arginine p-naphthylamide ( B A N A ) was not cleaved by either GB or SCTPA. BANA caused 50% inhibition of G B at 2 x 1 0 F J M when assayed with MUGB, and 50% inhibition of SCTPA at lo-’ M when assayed with substrate S-2288. We conclude that the proteinases SCTPA and GB (although not necessarily identical) are more closely related t o each other in their location, size, substrate specificity and inhibition characteristics than to the other trypsin-like proteinases.