Sensitive fluorometry of heat-stable alkaline phosphatase (Regan enzyme) activity in serum from smokers and nonsmokers.

We developed a simple, sensitive enzymatic assay involving the fluorogenic substrate naphthol AS-MX phosphate [(3-hydroxy-2-naphthoic acid 2,4-dimethylanilide) phosphate] to measure heat-stable alkaline phosphatase (EC 3.1.3.1), the Regan isoenzyme, in human serum. The day-to-day CV was 5.7% for a serum activity of 0.080 arbitrary units/L. Measurable amounts of enzyme were detected in most normal individuals. The mean for 51 nonsmokers was 0.068 (SD 0.037) arb. units/L; for 25 smokers it was 0.440 (SD 0.360) arb. units/L. Activity of this isoenzyme in smokers was as much as 10-fold the upper normal limit for nonsmokers. Activation of this tumor marker by smoking has not received attention hitherto. We conclude that a truly normal range can only be established among nonsmokers. The isoenzymes in smokers, nonsmokers, and pregnant women were similar in their heat stability, immunologic cross reactivity, and inhibition by L-phenylalanine.