Existence of the Cysteine Residue in a Region Close to Active Center of Human Paraoxonase1

다국어 초록 (Multilingual Abstract)

Paraoxonase1 (PON1), one of antioxidant proteins, is known to protect low density lipoprotein (LDL) from the oxidation. Despite no participation of cysteine residue in the catalysis of PON1, some part of its antioxidant action is accounted for by the sulhydryl group of the cysteine residue. Here, we attempted to elucidate the location of cystein residue in the active site. For this purpose, PON1 was exposed to various mercury compounds, which can mod& the cysteine residue, and the remaining activity was determined based on the hydrolysis of phenyl acetate. Of various mercury compounds, the inactivating potency was dependent on the size of mercury compounds; mercuric chloride>mercurochrome>mercury orange>p-hydroxymercuribenzoate. In order to confirm the modification of cysteine residue, each mercury compound-modified PON1 was exposed to excess DTT, and the restored activity was determined. Noteworthy, the the DTT reduction successfully restored the loss of PON1 activity, which was caused by the mercury compounds except p-hydroxymercuribenzoate, negatively-charged. This suggests that there may be a difference in the microenviroment of the active site among the mercury compound-modified PON1 molecules, consistent with the notion that the active site of PON1 molecule may contain the non-polar background. Probably in support of this, oleic acid, a protective fatty acid, effectively prevented against the inactivation of PON1 by p-hydroxymercuribenzoate, but not the other mercury compounds. Thus, the orientation of mercury compound-modified cysteine residue in the active site of PON1 could differ remarkably according to the type of mercury compound. Based on these results, it is suggested that the cysteines residue in the active site of PON1 may be oriented to be affected by extraneous factors.