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J. Biochem, 1977, Vol. 81, No. 4 1011-1016
© 1977 Japanese Biochemical Society

research-article

Some Heavy Metals Affecting the Lecithin-Cholesterol Acyltransferase Reaction in Human Plasma

Mitsuo NAKAGAWA, Masako TAKAMURA and Shoji KOJIMA

Faculty of Pharmaceutical Sciences, Kumamoto Univeisity Kumamoto, Kumamoto 862

The effects of cadmium (Cd2+) zinc (Zn2+), and calcium (Ca2+) ions on the esterification of cholesterol in sonicated dispersions of lecithin-cholesterol mixtures by lecithin-cholesterol acyltransferase (LCAT) in human plasma were studied in vitro.

The acyltransferase activity was slightly stimulated at concentrations of Ca2+ ions ranging from 1×10–5M to 1×10–3M but depressed at concentrations above 1×10–4M Cd2+ ions and above 5×10–4 M Zn2+ ions. The concentration of Zn2+ ions required for 50% inhibition of the acyltransferase activity was approximately twice that of Cd2+ ions. The acyltransferase inhibition caused by the addition of 1×l0–3 M Cd2+ or Zn2+ ions was completely overcome by the addition of 0.1% of ethylenediaminetetraacetic acid (EDTA) but not by the addition of 1×10–3M citrate, cysteine, mercaptoethanol or 22+ ions.

The decreased acyltransferase activity in the presence of 1×10–2 M Cd2+ ions increased linearly on the addition of increasing amounts of bovine serum albumin. The decreased acyltransferase activity in the presence of 1×l0–3 M Zn2+ (65.4 µg as Zn per ml of incubation medium) or Cd2+ (112.4 mu;g as Cd per ml of incubation medium) ions was restored to the control level or approximately 70% of the control level, respectively, by the addition of 40 mg of bovine serum albumin per ml of the incubation medium. Thus the molar ratio of Zn2+ binding to bovine serum albumin may be greater than that of Cd2+ binding.

In the presence of Cd2+ or Zn2+ ions, the acyltransferase activity toward a dispersion with a lecithin/cholesterol molar ratio of 5.9 was decreased more than that toward a dispersion with a ratio of 1.2. This result suggests that increasing amounts of lecithin in the dispersion added as a substrate may enhance the interfering effect of Cd2+ and Zn2+ ions on the acyltransferase.


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