Probable Involvement of Cathepsin D in the Degradation of β2-Microglobulin in Acidic Urine

Abstract Instability of β2-microglobulin in acidic urine was investigated by identifying an associated protease from normal urine. Degradation was completely blocked by pepstatin, an aspartic protease inhibitor, and the counterpart of the inhibitor was thus sought. The molecular weight of the counterpart was similar to that of the inhibitor, while its cleavage site on β2-microglobulin was identical in three products generated in purified β2-microglobulin in normal acidified urine (pH 5.0–5.5) and those generated by direct reaction between purified β2-microglobulin and cathepsin D in acetic acid (pH 5.0). On Western blotting, the presence of cathepsin D was demonstrated immunochemically in urine, and its urinary concentration correlated well with degree of β2-microglobulin degradation. All these findings strongly suggest that cathepsin D is a major urinary acid protease involved in the degradation of β2-microglobulin.