In the GM2 gangliosidosis B1 variant, the mutated isoenzyme A of beta-hexosaminidase (Hex) is incapable of hydrolyzing ganglioside GM2 and negatively charged substrates. Biochemical characterization of this lysosomal disease is carried out using synthetic alpha-subunit-specific sulfated substrates, as heat-inactivation assays are not applicable. The apparent enzyme activation energy of Hex using the chromogenic substrate 3,3'-dichlorophenolsulfonphthaleinyl N-acetyl-beta-D-glucosaminide is related directly to the relative proportions of Hex A and Hex B isoenzymes. This thermodynamic variable was used for the study of Hex enzyme heterogeneity in 3 patients with the GM2 gangliosidosis B1 variant and 6 heterozygote carriers. Hex activity was determined at 25 degrees C, 30 degrees C, 35 degrees C, and 37 degrees C in a Cobas Bio analyzer (Roche Diagnostics, Basel, Switzerland), and Arrhenius plot slopes and apparent activation energies were calculated in plasma samples and mononuclear and polymorphonuclear leukocyte lysates. The determination of the Hex isoenzymes in plasma presented a high discrimination power for B1 variant patients but not for heterozygote carriers, in whom false-negative results may be obtained. However, thermodynamic evaluation of the isoenzyme composition of Hex in leukocyte lysates permits the biochemical identification of patients with the GM2 gangliosidosis B1 variant and of heterozygote carriers.