Identification by nuclear magnetic resonance and mass spectrometry of a glucuronic acid conjugate of o-hydroxybenzoic acid in normal urine and uremic plasma.

An endogenous compound (included in the fraction of uremic toxins often called the "uremic middle molecules") was separated from plasma of uremic patients and urine from normal persons. As elucidated by mass spectrometry, enzymatic hydrolysis, and 1H, 13C nuclear magnetic resonance, it appears to be a conjugate of o-hydroxybenzoic acid with glucuronic acid. Its presence in urine of healthy subjects indicates its physiological character.