J Bone Miner Res 2003 Oct;18(10):1916-9
Department of Veterans Affairs Medical Center, Louisville, Kentucky 40206, USA.
The association between elevated serum type 5 TRACP activity and metabolic bone diseases has been recognized for many years. However, serum type 5 TRACP exists as two related isoforms: 5a and 5b. Only isoform 5b is osteoclast-derived; the origin and significance of isoform 5a has hardly been explored. We have used simultaneous immunoassays for type-5 TRACP activity and total type-5 TRACP protein in conjunction with non-denaturing gel electrophoresis and column chromatography to investigate the nature and significance of TRACP isoforms 5a and 5b in end-stage renal disease (ESRD) and rheumatoid arthritis (RA). Our studies have shown that TRACP activity and protein are elevated in approximately 50% of sera from ESRD patients, which is caused by osteoclastic isoform 5b. We have also shown that total TRACP protein, but not TRACP activity, is elevated in approximately 30% of sera from RA patients, which is caused by non-osteoclastic isoform 5a. When macrophages or dendritic cells (DC) were cultured in vitro, abundant TRACP 5a was secreted into the culture medium, whereas TRACP 5b was retained intracellularly by both cell types. This implicates macrophages and DC as potential sources of elevated TRACP 5a in RA. Because TRACP isoform expression may be disease-specific, it is important to be able to distinguish TRACP 5a from 5b. There are four criteria by which to do so: (1) TRACP 5a bears sialic acid residues while TRACP 5b does not; (2) the pH optimum for TRACP 5a is 5.2 while that for TRACP 5b is 5.8; (3) the specific activity of TRACP 5a is significantly lower than that of TRACP 5b; and (4) TRACP 5a is as an uncleaved polypeptide, whereas TRACP 5b is a proteolytically nicked disulfide-linked "heterodimer." The differences in biochemical properties and disease-specific expression of TRACP isoforms 5a and 5b suggest that they are regulated differently and perform separate functions in a tissue-specific manner.