Thermodynamic characterisation of the mutated isoenzyme A of beta-N-acetylhexosaminidase in GM2-gangliosidosis B1 variant

Hexosaminidase assays

beta-Hexosaminidases (EC 3.2.1.52) are lysosomal enzymes that remove terminal beta-glycosidically bound N-acetylglucosamine and N-acetylgalactosamine residues from a number of glycoconjugates. Reliable assay systems are particularly important for the diagnosis of a family of lysosomal storage disorders, the GM2 gangliosidoses that result from inherited beta-hexosaminidase deficiency. More recently, aberrant hexosaminidase levels have also been found to be associated with a variety of inflammatory diseases. Apart from patient testing and carrier screening, practical in vitro assays are indispensable for the characterization of knock-out mice with potentially altered hexosaminidase activities, for detailed structure-function studies aimed at elucidating the enzymatic mechanism, and to characterize newly described enzyme variants from other organisms. The purpose of this article is to discuss convenient hexosaminidase assay procedures for these and other applications, using fluorogenic or chromogenic artificial substrates as well as the physiological glycolipid substrate GM2. Attempts are also made to provide an overview of less commonly used alternative techniques and to introduce recent developments enabling high-throughput screening for enzyme inhibitors.

β-Hexosaminidase isoenzyme profiles in serum, plasma, platelets and mononuclear, polymorphonuclear and unfractionated total leukocytes

OBJECTIVES

The relative proportion in percentage of the isoenzyme A of beta-hexosaminidase (Hex) is the single discriminatory function most frequently used for the biochemical screening of heterozygote Tay-Sachs disease carriers. It has been suggested that the assay of the Hex isoenzymes in homogeneous cell preparations is preferable to that in mixed total leukocytes which present greater interindividual variation. The major aim of our study was the evaluation of this hypothesis.

DESIGN AND METHODS

Total Hex and its Hex A and Hex B isoenzymes were determined in different samples of serum and plasma (n = 81) as well as in lysates of platelets (n = 75), and mononuclear (n = 81), polymorphonuclear (n = 81) and mixed total leukocytes (n = 33).

RESULTS

The interindividual variations found for % Hex A in the different biological samples were: plasma (CV = 23.4%), platelets (CV = 10.2%), mononuclear (CV = 5.7%), polymorphonuclear (CV = 5.3%) and total leukocytes (CV = 7.1%). Although the relative proportion of Hex A was significantly greater in polymorphonuclear than in mononuclear leukocytes (P < 0.001), a statistical significance was not attained for the correlation between the relative proportions of blood polymorphonuclear cells and Hex A in mixed total leukocytes (r = 0.220).

CONCLUSIONS

The use of total leukocyte lysates does not appear to introduce a significant increase for the interindividual variation of the Hex A isoenzyme relative proportion in relation to the use of homogeneous cell preparations.

Biochemical characterization of the GM2 gangliosidosis B1 variant

The deficiency of the A isoenzyme of beta-hexosaminidase (Hex) produced by different mutations of the gene that codes for the alpha subunit (Tay-Sachs disease) has two variants with enzymological differences: the B variant consists of the absence of Hex A isoenzyme and the B1 variant produces an inactive Hex A isoenzyme for the hydrolysis of the GM2 ganglioside and synthetic substrates with negative charge. In contrast to the early childhood form of the B variant, the B1 variant appears at a later clinical stage (3 to 7 years of age) with neurodegenerative symptoms leading to the death of the patient in the second decade of life. The most frequent mutation responsible for the GM2 gangliosidosis B1 variant is R178H, which has a widespread geographic and ethnic distribution. The highest incidence has been described in Portugal, which has been suggested as the point of origin of this mutation. Biochemical characterization of this lysosomal disease is carried out using negatively charged synthetic alpha subunit-specific sulfated substrates, since Hex A isoenzyme heat-inactivation assays are not applicable. However, the determination of the apparent activation energy of Hex using the neutral substrate 3,3'-dichlorophenolsulfonphthaleinyl N-acetyl-beta-D-glucosaminide, may offer a valid alternative. The presence of an alpha subunit in the alphabeta heterodimer Hex A means that its activation energy (41.8 kJ/mol) is significantly lower than that of the betabeta homodimer Hex B (75.1 kJ/mol); however, as mutation inactivates the alpha subunit, the Hex A of the B1 variant presents an activation energy that is similar to that of the Hex B isoenzyme.

Thermodynamic determination of plasma and leukocyte beta-hexosaminidase isoenzymes in homozygote and heterozygote carriers for the GM2 gangliosidosis B1 variant

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.

Effect of partial proteolysis on the activation energy of beta-n-acetylhexosaminidase precursor and mature forms

Using 3,3'-diclorophenolsulfoftaleinil N-acetyl-beta-D-glucosaminide as a substrate, the apparent activation energy of beta-N-acetylhexosaminidase (Hex) was determined in samples of plasma and urine, as well as in leukocyte and platelet lysates. Incubation with papain produced an increase in this thermodynamic variable for plasma Hex (precursor forms with high molecular mass) that would be caused by the proteolytic action of papain on the Hex A isoenzyme. However, digestion with papain did not significantly modify the activation energy of Hex in leukocyte and platelet lysates (mature enzymatic forms). In 11 healthy subjects and 28 patients with different renal diseases, no statistically significant differences were found with regard to the values obtained in cellular lysates for variations in the activation energy of urinary Hex, regardless of whether they presented normoalbuminuria, microalbuminuria or macroalbuminuria. These results support the hypothesis that even in patients with proteinuria, no significant amounts of plasma Hex precursor forms are found in urine samples, and the source of the enzyme activity is the kidney itself.

Thermodynamic determination of beta-hexosaminidase isoenzymes in mononuclear and polymorphonuclear leukocyte populations

Isoenzymes of beta-hexosaminidase (Hex) were determined in mononuclear (MN) and polymorphonuclear (PMN) leukocytes, with a thermodynamic method using the chromogenic substrate sodio-3,3'-dichlorophenolsulfonphthaleinyl N-acetyl-beta-D-glucosaminide. Imprecision was very satisfactory, and the results are very much in agreement with those obtained using the fluorogenic substrates 4-methylumbelliferyl N-acetyl-beta-D-glucosaminide and 4-methylumbelliferyl N-acetyl-beta-D-glucosaminide 6-sulfate. In 163 healthy individuals we found, for the proportion as a percentage of the Hex A isoenzyme, significantly higher values (P < 0.001) in PMN than in MN cells (71.56 +/- 0.30% vs. 54.28 +/- 0.24%), meaning that it would not appear advisable to use total leukocyte lysates for evaluating this variable. The method is fast, precise, and highly suitable for the biochemical diagnosis and heterozygote screening of GM2 gangliosidoses, and would be applicable in cases of thermolabile Hex B and for detecting the B1 variant.

Plasma beta-N-acetylhexosaminidase isoenzyme composition and temperature conversion factors

Using the chromogenic substrate 3,3'-dichlorophenylsulfonphthaleinyl-N-acetyl-beta-D-glucosaminide for the activity determination of plasma beta-N-acetylhexosaminidase (Hex), the temperature conversion factors (TCF) offer a highly significant positive correlation with the relative proportion of Hex B isoenzyme (P< 0.001). The calculation of TCF 37 degrees/25 degrees C allows the isoenzyme composition of Hex to be determined quickly and cheaply. The results may be superimposed over those obtained in a previously described method based on the calculation of the enzyme's activation energy using four temperatures. However, the use of TCF 37 degrees/30 degrees C does not appear to comply with the required demands.

Relationship between plasma ammonia concentration and beta-N-acetylhexosaminidase isoenzyme activities in liver cirrhosis

Ammonia is known to increase the secretion of beta-N-acetylhexosaminidase (Hex) to the extracellular medium in cultured human fibroblasts and Hep G2 cells. We examined 35 patients with liver cirrhosis and the results revealed a significant increase for the plasma activities of total Hex and its isoenzymes Hex A and Hex B (p < 0.001). The partial correlations, with other biochemical markers of liver injury constant, between plasma ammonia concentration and the activity (r = 0.658) and the proportion in percentage of the Hex B isoenzyme (r = 0.692) were statistically significant (p < 0.001). The increased concentration of ammonia could explain, at least partially, an increased secretion of Hex B isoforms to the plasma in patients with liver disease.