Materials and Methods.-FDPase was purified according to the procedure of Pontremoli et al. 3 The enzyme activity was measured spectrophotometrically at 340 m1u by following the rate of production of fructose 6-phosphate in the presence of triphosphopyridine nucleotide (TPN), excess phosphoglucose isomerase, and glucose 6-phosphate dehydrogenase. The assay systems (1ml) contained 0.1 mM fructose 1, 6-diphosphate (FDP), 1 mM MnCl2 or 10 mM MgCl2, 0.1 mM TPN, 0.3 units of glucose6-phosphate dehydrogenase, 2 units of hexosephosphate isomerase, and either 40 mM triethanolamine buffer, pH 7.5, containing 0.1 mM ethylenediaminetetraacetate (EDTA), or 40 mM glycine buffer, pH 9.1. The temperature inthe spectrophotometer chamber was 220. The unit of enzyme activity was defined as the quantity required to produce an absorb-ance change of1. 0 unit per minute under these conditions. Specific activity is expressed as units per milligram of protein. The protein concentration was determined by the phenol method'2 standardized by dry weight determination of a sample of dialyzed crystalline FDPase. Glucose 6-phosphate dehydrogenase, hexosephosphate isomerase, and glutathione reductase were purchased from Boehringer-Mannheim, Germany. 6-Phosphogluconate dehydrogenase was prepared as previously described.'Cystamine dihydrochloride, pantethine, lipoic acid, lipo-amide, and Coenzyme A were purchased from Sigma Chemical Corporation; 5, 5'-dithio bis (2-nitrobenzoic) acid fromK and K Laboratories; and 2-hydroxyethyl disulfide, dithiodiglycollic acid, and 3-carboxypropyl disulfide from Aldrich Chemical Co. Sulfhydryl groups were measured spectrophotometrically by titration with p-mercuribenzoate