Estradiol esterification in the human preovulatory follicle

In the preovulatory follicle, the LH surge stimulates progesterone producti on, reduces estradiol synthesis, and scales up the permeability of the bloo d-follicle barrier. The purpose of this study was to investigate whether th e extent of these changes is correlated with the levels of estradiol, estra diol esters, and cholesteryl esters in the follicular fluid. The follicular levels of progesterone, estradiol, estradiol linoleate, cholesterol, and c holesteryl linoleate were measured by HPLC. The estradiol linoleate/estradi ol ratio, which reflects the efficiency of in vivo estradiol esterification , and the cholesteryl linoteate/cholesterol ratio were calculated and found negatively correlated. The estradiol. level was positively correlated with the cholesteryl linoleate/cholesterol ratio while negatively correlated wi th the estradiol linoleate/estradiol ratio. The in vitro activity of lecith in-cholesterol acyltransferase, the enzyme esterifying both cholesterol and estradiol, was assayed by incubating the fluid with labeled substrates. Th is activity was not correlated with either the estradiol linoleate/estradio l or the cholesteryl linoleate/cholesterol ratio. The enzyme K-m and V-max. values were lower with estradiol than with cholesterol. Higher estradiol l inoleate/estradiol ratios and lower cholesteryl linoleate/cholesterol ratio s were associated with higher level of Haptoglobin penetration into the fol licle. This level, which was determined by ELISA, was found increased with increased progesterone concentration and, therefore, used as a marker of th e LH-stimulated permeability of the blood-follicle barrier. Our data sugges t that early preovulatory follicles contain more cholesteryl esters and les s estradiol esters than follicles closer to ovulation. (C) 2001 Elsevier Sc ience Inc. All rights reserved.