Haptoglobin inhibits lecithin-cholesterol acyltransferase in human ovarianfollicular fluid

The activity of the enzyme lecithin-cholesterol acyltransferase (LCAT; E.C. 2.3.1.43) is involved in the removal of cholesterol excess from peripheral cells. This activity is stimulated by the HDL (high density lipoprotein) a polipoprotein A1 (ApoA1). Haptoglobin (Hpt) was previously found to be asso ciated with ApoA1 in ovarian follicular fluid. LCAT activity was analyzed i n follicular fluids, collected from an IVF program, containing different am ounts of Hpt or Hpt/ApoA1 ratio. Addition of purified Hpt to follicular flu id caused a decrease in the enzyme activity, which was measured as the rate of synthesis of cholesteryl esters. In the fractions of fluid proteins, as obtained by gel filtration chromatography, Hpt and HDL were titrated by EL ISA while the LCAT activity was assayed by using radioactive cholesterol an d purified HDL. When isolated LCAT was incubated with fractions containing different Hpt/ApoA1 ratios, the enzyme activity was found negatively correl ated with the Hpt/ApoA1 ratio (P < 0.01). LCAT kinetic parameters were meas ured in two fractions with the same amount of ApoA1 (5 <mu>g/ml) but differ ent amounts of Hpt (0.69 or 3.77 mug/ml): the V-max did not change while th e K-m values were 24.1 or 78.6 muM in the presence of the low or high Hpt l evel, respectively. The analysis of fluids associated with cytoplasmically mature MII oocytes, in a cross-sectional study, confirmed that a negative c orrelation exists between the Hpt/ApoA1 ratio and the LCAT activity (P < 0. 01). The results suggest that Hpt inhibits the reverse transport of cholest erol by preventing ApoA1 stimulation of the LCAT activity. Mol. Reprod. Dev . 59:186-191, 2001. (C) 2001 Wiley-Liss, Inc.