Diurnal fluctuations of protein contents and the pH dependence of β2-microglobulin stability in urine

Diurnal fluctuations of protein excretion into urine and the effect of urinary pH on the urinary protein concentrations were studied in patients with various kidney diseases. The diurnal kinetics of γ-immunoglobulin, transferrin, albumin, α1-microglobulin, γ-immunoglobulin light chains, and the retinol-binding protein proved to positively correlate with the diurnal fluctuations of proteinuria and to negatively correlate with urinary pH. Diurnal changes in urinary β2-microglobulin content did not correlate with those of any other protein. Oral bicarbonate intake alkalinized the urine, increased the urinary β2-microglobulin content, and led to a direct correlation between β2-microglobulin excretion and excretion of other low-molecular proteins. Thus, proteinuria, single protein excretion, and urinary pH displayed diurnal rhythmicity in the patients; β2-microglobulin was unstable in acid urine and its urinary level depended on the urinary pH.