Glycyiglycyiglycineas a Synthetic Standard for Serum Protein Determination1

Glycylglycyiglycine was investigated as a reference standard for use in serum protein measurement by the biuret reaction. The tripeptide-biuret solution has a molar absorptivity of 96 at 565 nm, and absorbances at both 550 nm and 565 nm are proportional to concentration. By a manual reference procedure, the 550-nm absorbance of 1.0 g of tripeptide was equivalent to that given by 1.72 ± 0.03 g of human serum albumin or 1.43 ± 0.03 g of bovine serum albumin. By the Technicon N14b automated procedure, the absorbance of 1.0 g of tripeptide at 550 nm was equivalent to that of 1 .81 ± 0.02 g of human serum albumin or 1.89 ± 0.03 g of bovine serum albumin. Results for serum protein analyses over the range 4.0 to 9.0 g/dl, when tripeptide or serum albumin was used to prepare calibration curves, showed mean differences of 0.15 g/dl in the manual mode and 0.08 g/dl in the automated mode.