Enzymes of lysosomal origin in human plasma and serum: assay conditions and parameters influencing the assay

The conditions for maximal activity (pH, buffer, saturating substrate concentration, range of linear relationships between enzyme activity versus incubation time, and versus enzyme concentration) in the fluorimetric assay of several glycohydrolases of lysosomal origin in human plasma and serum have been established. The following enzymes were studied: α-galactosidase, β-galactosidase, β-N-acetylglucosaminidase, β-glucosidase, β-glucuronidase, α-mannosidase, α-fucosidase. All examined enzymes turned out to be more or less unstable upon storage at 37°C, 4°C, and -20°C in both serum and plasma. The only exceptions were β-glucuronidase, which was stable in plasma and serum, and α-fucosidase which was stable only in plasma. Generally the degree of instability was greater in serum than in plasma. The levels of some enzymes α-galactosidase, β-galactosidase, β-N-acetyl glucosaminidase, β-glucuronidase) were markedly higher in serum than in plasma; conversely the levels of the same enzymes in "platelet free" serum equalled those in plasma. This stresses the necessity to use freshly prepared plasma for lysosomal glycohydrolase assay. Under the procedural conditions recommended for the assay the methods for the determination of lysosomal glycohydrolases in plasma appeared to be simple, sensitive and reproducible.