An ingredient in human semen may actually help HIV infect cells, German researchers say.

Dr Jan Münch of the University Clinic of Ulm and colleagues report their findings in the current issue of the journal Cell.

They say a naturally ocurring enzyme produced by the prostate, called prostatic acidic phosphatase (PAP), can form tiny fibres that capture bits of HIV and usher it into cells.

The researchers made the discovery while looking for factors in semen that might block infection with HIV-1, the most common strain of the virus that causes AIDS.

Instead, they found one that enhanced transmission, in some experiments as much as 50-fold.

"In this study we show that fragments of PAP, a highly abundant semen marker, form amyloid fibrils [fibres] that drastically enhance the infectiousness of HIV-1," the researchers write.

They say they are now looking for compounds that might block this process to slow transmission of the disease.