Apolipoprotein D and alpha 1-microglobulin in human urine: effect of cadmium exposure

Apolipoprotein D is a previously unrecognized urinary protein of unknown function which we have tested as a potential marker for kidney malfunction. This protein and alpha 1-microglobulin have been quantified by zone immunoelectrophoresis assay in urine samples from a group of eight men occupationally exposed to cadmium-containing welding fumes for many years. All these workers had highly elevated concentrations of urinary cadmium and indications of tubular proteinuria, as compared to a group of 50 apparently healthy normal men analyzed in parallel. The cadmium-exposed workers demonstrated three- and 15-fold average increases in apolipoprotein D and alpha 1-microglobulin, respectively, over normal values in urine, estimated both as excretion rates and as milligrams of protein per mmol of creatinine. All these increments were highly significantly different (P < 0.001) from the corresponding values of the reference group. Essentially the same results were obtained for each of the proteins from two independent consecutive samplings of the workers' urine. There were good linear (R = 0.70, 0.80) and logarithmic (R = 0.84, 0.81) correlations between the urinary concentrations of alpha 1-microglobulin and apolipoprotein D for both the reference and the study group. Although not as sensitive an indicator for tubular proteinuria as alpha 1-microglobulin, apolipoprotein D, being a storage-stable urinary protein, seems a valuable complement for the diagnosis of tubular malfunction.