Sulfhydryl groups of rabbit liver arylsulfatase A

Chemical modification of NADP-isocitrate dehydrogenase from Cephalosporium acremonium evidence of essential histidine and lysine groups at the active site

NADP-isocitrate dehydrogenase from Cephalosporium acremonium CW-19 has been inactivated by diethyl pyrocarbonate following a first-order process giving a second-order rate constant of 3.0 m-1. s-1 at pH 6.5 and 25 degrees C. The pH-inactivation rate data indicated the participation of a group with a pK value of 6.9. Quantifying the increase in absorbance at 240 nm showed that six histidine residues per subunit were modified during total inactivation, only one of which was essential for catalysis, and substrate protection analysis would seem to indicate its location at the substrate binding site. The enzyme was not inactivated by 5, 5'-dithiobis(2-nitrobenzoate), N-ethylmaleimide or iodoacetate, which would point to the absence of an essential reactive cysteine residue at the active site. Pyridoxal 5'-phosphate reversibly inactivated the enzyme at pH 7.7 and 5 degrees C, with enzyme activity declining to an equilibrium value within 15 min. The remaining activity depended on the modifier concentration up to about 2 mm. The kinetic analysis of inactivation and reactivation rate data is consistent with a reversible two-step inactivation mechanism with formation of a noncovalent enzyme-pyridoxal 5'-phosphate complex prior to Schiff base formation with a probable lysyl residue of the enzyme. The analysis of substrate protection shows the essential residue(s) to be at the active site of the enzyme and probably to be involved in catalysis.

Inability of thiol compounds to restore CNS arylsulfatases inhibited by methyl mercury

Arylsulfatase A and B activities have been analysed in different areas of the CNS (olfactory bulbs, cerebral hemispheres, cerebellum, medulla oblongata and spinal cord) during methyl mercuric chloride (MMC), N-acetyl-DL-homocysteine thiolactone (NAHT) and glutathione (GSH) treatment. Male albino rats were administered with two different doses of MMC (1 mg/kg and 10 mg/kg), NAHT (40 mg/kg and 80 mg/kg) and GSH (100 mg/kg and 150 mg/kg) for 2, 7 and 15 days, and sacrificed on the 3rd, 8th and 16th day, respectively. Likewise, eight groups of animals, after treatment for 7 days with MMC, were kept for another 7 days and sacrificed on the 15th day (normal withdrawal). Sixteen groups of 7 days MMC pretreated animals were given either NAHT or GSH for another seven days before sacrifice on the 15th day. The study revealed 1) a dose and duration dependent inhibition of the enzymes in all CNS areas, 2) a maximum inhibition of the enzymes among various hydrolases reported so far, and 3) none of the antagonists restored the enzymes significantly except for one group with NAHT. It is, therefore, concluded that arylsulfatases are more sensitive to MMC than other lysosomal enzymes, and the antagonists used here have not helped their restoration.