Preparation of a stable liquid material for calibration and quality control for lysosomal enzymes in plasma: Assay of enzymes of lysosomal origin in plasma, I

Several lysosomal enzymes present in human plasma (N-acetyl-β-glucosaminidase, β-glucuronidase, β-galactosidase, α-galactosidase, α-L-fucosidase, α-mannosidase, β-glucosidase) were maintained in a fully active state for at least 8 months by the addition of ethylene glycol (300 g/l final concentration) to freshly prepared plasma and storage at -20°C. Pools of human plasma from healthy humans, stabilized and stored as above, and containing a low, medium or high content of the above enzymes, were used to establish the analytical imprecision (within-run, day-to-day and total imprecision) of the fluorimetric assay. Ten replicates in ten different analytical series, covering a period of two months, were performed. The total imprecision (expressed as coefficient of variation) was in general lower than 10%; in a few cases, particularly plasma samples with a low enzyme content, the total imprecision was 18%. The isozymes A, B, I₁, and I₂ of N-acetyl-β-glucosaminidase displayed the same stability upon storage as the unfractionated enzyme. It is concluded that pools of human plasma containing known amounts of lysosomal enzymes, stabilized by the addition of 300 g/l ethylene glycol and stored at -20°C, are suitable liquid materials for calibration and quality control for the assay of the same enzymes.