Characterization of the soluble, secreted form of urinary meprin

A soluble form of the kidney membrane metalloendopeptidase, meprin, is present in urine. Urinary meprin is expressed in BALB/C mice with the Mep-1(alpha/alpha) genotype (high meprin, expressing meprin-alpha and meprin-beta) but not in BALB.K mice of the Mep-1(b/b) genotype (that only express meprin-beta). Western blotting with antisera specific to the meprin-alpha and the meprin-beta subunits established that the only form of meprin present in urine samples was derived from meprin-alpha. This form of meprin is partially active, and comprises at least three variants by non-reducing SDS/PAGE and by zymography and two protein bands on reducing SDS/PAGE. Sequencing of these two bands established that the N-terminus of the larger protein band begins with the pro-peptide sequence of the alpha-subunit (VSIKH..), whereas the smaller band possessed the mature meprin N-terminal sequence (NAMRDP..). Trypsin is able to remove the pro-peptide, with a concomitant activation in proteolytic activity. After deglycosylation, the size of the pro- and mature forms of urinary meprin are consistent with cleavage in the region of the X-I boundary. There is a pronounced sexual dimorphism in urinary meprin expression. Females secrete a slightly larger form, and its proteolytic activity is about 50% of that released by males. The urinary meprin is therefore a naturally occurring secreted form of this membrane-bound metalloendopeptidase and is more likely to be generated by alternative processing pathways than by specific release mechanisms.