Enzymatic assays of l-serine in biological material

Methods are presented for highly specific enzymatic determinations of l-serine in biological material. The methods are based on the use of partially purified pyrophosphate:l-serine O-phosphotransferase (EC 2.7.1.80) to convert l-serine to l-phosphoserine which, in turn, is assayed spectrophotometrically or fluorometrically using the combined reactions of l-phosphoserine aminotransferase (EC 2.6.1.52) and d-3-phosphoglycerate dehydrogenase (EC 1.1.1.95). Since as little as 0.2 nmol of l-serine can be assayed, the sensitivity of the methods compares favorably with that of an amino acid analyzer. Using the assays, values for free l-serine have been determined in freeze-clamped, normal, fed rat tissues to be 0.77 ± 0.04 μmol/g (liver); 0.72 ± 0.04 μmol/g (brain); 0.73 ± 0.04 μmol/g (kidney) and 0.52 ± 0.11 μmol/g (muscle).