Rapid changes in urinary serine and branched-chain amino acid excretion among diabetic patients during insulin treatment.

In a preliminary experiment, we found a good correlation between 24-h urinary amino acid excretion and the 24-h average plasma levels of the same amino acids. Examining diabetics who were just beginning insulin therapy, we found that insulin normalized the abnormally high levels of excretion of branched-chain amino acids and serine. Interestingly, when expressed in terms of mol/g of creatinine, the normalization of serine excretion brought about by insulin was roughly equal to the normalization of glycine excretion brought about by insulin (-0.39 mM/g of creatinine vs. + 0.33 mM/g of creatinine over 24 h). Since plasma serine is primarily produced in the kidneys from glycine, this suggests that insulin affects the regulation of the serine-glycine metabolic pathway. In turn, measurement of urinary serine and glycine may provide a useful gauge of insulin activity in the tissues, including the kidneys.