BALTIMORE (AP) _ A study in mice suggests that an enzyme in the brain plays a key role in brain damage from stroke, boosting the idea that blocking this substance might reduce stroke damage in people.

The work also might provide insights into such conditions as Alzheimer's and Parkinson's diseases.

The enzyme normally repairs DNA. Researchers at the Johns Hopkins University School of Medicine suggest that a stroke throws this enzyme into overdrive, leading to cell death.

The enzyme, called PARP, previously had been suggested as playing a critical role in brain cell destruction after stroke. Scientists hope to develop a drug that can be given after a stroke to block PARP's effects, and so limit brain cell death.

The new work is reported in the October issue of the journal Nature Medicine by Valina Dawson, Dr. Solomon Snyder and Richard Traystman and other scientists at Hopkins and elsewhere.

They studied the effect of PARP by creating a strain of mice that lacked it. Then they temporarily interrupted the blood supply to part of the animals' brains to simulate a stroke.

The mice that lacked PARP showed about 80 percent less brain cell destruction than other mice did.

``We replicated the experiment several times to convince ourselves that this was real,'' Ms. Dawson said.