Isolation and N-terminal sequence of multiple forms of granulins in human urine

Three molecular forms of granulins (also known as epithelins) were isolated, for the first time, in human urine. Their N-terminal sequences, which have also been determined, are identical to those of granulins A and B, previously isolated from human leukocytes, and of granulin F, never isolated before but whose primary structure is known on the basis of the cDNA sequence. The urinary molecules, which show a molecular weight of about 6.5 kDa, are most likely produced by a posttranslational proteolytic processing occurring at the level of the kidney, which appears to be the organ richest in granulin precursor mRNA. The molecular events underlying the precursor processing are unknown, even though the involvement of the protease kallikrein, an enzyme thought to be responsible for the processing of several polypeptidic growth factor precursors, could be hypothesized. Granulins, however, do not show antikallikrein activity. The presence in human urine of isoform F, previously not identified from other human sources, seems to support the hypothesis that mature forms of granulins are generated by an organ-specific precursor processing, on the basis of particular physiological requirements, and to suggest also that this isoform may play "in vivo" an important and specific role in the epithelial cells of the human kidney.