An enzymatic sex difference.

Why do so many men suffer from heart attacks earlier in life than women do? A new report hints that an enzyme in the blood may help explain the difference.

John E. Hokanson of the University of Washington, Seattle and his colleagues studied 25 men and 39 women, all of whom had normal concentrations of cholesterol--including low-density lipoprotein (LDL), the so-called bad cholesterol-in their blood. The researchers took samples of the volunteers' blood and measured the activity of an enzyme called hepatic lipase. They discovered that hepatic lipase activity was just over 50 percent higher in men than in women.

The researchers also found that the men had significantly lower concentrations of high-density lipoprotein, or good cholesterol, as well as more of a particular type of bad cholesterol--the small, dense LDL thought to be the worst of the bad cholesterol.

Hokanson and his colleagues believe that higher concentrations of hepatic lipase lead to the unfavorable lipid profile seen more commonly in men. Estrogen, the primary female sex hormone, appears to regulate hepatic lipase, leading the researchers to speculate that estrogen keeps concentrations of hepatic lipase low in premenopausal women.

After menopause, a woman's risk of heart disease starts to rise. Hokanson suggests that this risk is related to the increase in hepatic lipase enzyme. Hokanson presented the team's findings on March 20 at a meeting of the American Heart Association in Santa Fe, N.M.