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Effects of methyl mercury and cadmium on the kinetics of substrate activation of (K + )‐paranitrophenyl phosphatase
Author(s) -
Sahib K. I. Ahammad,
Moorthy K. S.,
Desaiah D.
Publication year - 1987
Publication title -
journal of applied toxicology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.784
H-Index - 87
eISSN - 1099-1263
pISSN - 0260-437X
DOI - 10.1002/jat.2550070313
Subject(s) - chemistry , dithiothreitol , phosphatase , metal , uncompetitive inhibitor , kinetics , cadmium , substrate (aquarium) , stereochemistry , non competitive inhibition , dephosphorylation , enzyme , enzyme kinetics , mercury (programming language) , active site , medicinal chemistry , biochemistry , organic chemistry , physics , oceanography , programming language , quantum mechanics , computer science , geology
Previous studies from this laboratory have indicated that methyl mercuric chloride (CH 3 HgCI) and cadmium chloride (CdCI 2 ) are potent inhibitors of K + ‐p‐nitrophenyl phosphatase (K + ‐PNPPase). The present studies were undertaken to study the effects of CH 3 HgCI and CdCI 2 on the substrate activation kinetics of K + ‐PNPPase to understand the mechanism of inhibition of Na + pump by these heavy metals. Uncompetitive inhibition with regard to activation by PNPP was indicated by altered V max and K m values by both the heavy metals. Substrate activation kinetics of heavy metal inhibited K + ‐PNPPase in the presence of 25 μM dithiothreitol and glutathione indicated mixed type of activation by altering apparent V max and K m . Absence of competition between PNPP site and heavy metals appear to indicate absence of reactive‐SH groups in the active site. Failure of added iodoacetate, in concentrations ranging from 5 × 10 −8 to 5 × 10 −5 M, to inhibit K + ‐PNPPase further substantiate this conclusion. The results suggest that CH 3 HgCI and CdCI 2 inhibit Na + pump by inducing conformational changes in the enzyme and thereby decrease catalytic velocity of dephosphorylation of the enzyme‐phosphoryl complex. Hydrolysis of PNPP was linear with time with or without either heavy metal and the inhibition exerted by CH 3 HgCI or CdCI 2 on free or heavy metal loaded enzyme indicated absence of heavy metal interaction. The results suggest that CH 3 HgCI and CdCI 2 inhibit K + ‐PNPPase possibly by binding at two different sites.