Stimulation of myosin light-chain kinase by Cd2+ and Pb2+.
The effect of Cd2+ on myosin light chain kinase (MLCK) reported in the literature is controversial, apparently because the level of Ca2+ contaminating the reaction mixture could not be accurately controlled by the addition of a metal chelator when Cd2+ was also present. In the present study, we have reduced the contaminating Ca2+ to a trace level that did not interfere with the enzyme activity; thus the use of a metal chelator was not necessary. We showed that Cd2+, or Pb2+ had a biphasic effect on MLCK isolated from chicken gizzard: stimulation at low and inhibition at high concentrations. (The stimulatory effect of on the enzyme activity isolated from chicken gizzard: stimulation at low and inhibition at high concentrations). The stimulatory effect of Cd2+ or Pb2+ on MLCK activity was not seen in the absence of calmodulin, and was abolished by trifluoperazine, a calmodulin antagonist, indicating that the heavy metals exert their activation via calmodulin. The inhibition of the enzyme activity by Cd2+ or Pb2+ at higher concentrations was also seen with the calmodulin-independent catalytic fragment of MLCK, suggesting that the inhibition is probably through their binding to sulfhydryl groups that are essential for catalytic activity. Pb2+ was more effective than Cd2+ in stimulating the enzyme activity, but less potent in inhibition. The extent of stimulation by heavy metals most likely resulted from a combination of the biphasic effects. Dithiothreitol and N,N,N',N'-tetrakis (2-pyridylmethyl) ethylenediamine selectively chelated Cd2+ and Pb2+ over Ca2+, and reversed their stimulatory or inhibitory effect on MLCK.(ABSTRACT TRUNCATED AT 250 WORDS)
Chao
S H
SH
Department of Biochemistry, St Jude's Children's Hospital, Memphis, TN 38101, USA.
Bu
C H
CH
Cheung
W Y
WY
D000818:Animals; D001013:Aorta, Thoracic / Q000187:drug effects; D002104:Cadmium / Q000494:pharmacology*; D002614:Chelating Agents / Q000494:pharmacology; D002645:Chickens; D005895:Gizzard, Avian / Q000187:drug effects; D066298:In Vitro Techniques; D007854:Lead / Q000494:pharmacology*; D009219:Myosin-Light-Chain Kinase / Q000187:drug effects* / Q000378:metabolism; D011817:Rabbits; D014661:Vasoconstriction / Q000187:drug effects
D002614:Chelating Agents; D002104:Cadmium; D007854:Lead; D009219:Myosin-Light-Chain Kinase