The chemicals – lupeol (found in mangoes and dandelion roots) and pristimerin (from the ancient anti-fertility herb known as 'thunder god vine') – prevent human sperm from whipping their tails and propelling into the egg, a key step in fertilisation.
'Because these two plant compounds block fertilisation at very, very low concentrations – about 10 times lower than levels of levonorgestrel in Plan B – they could be a new generation of emergency contraceptive we nicknamed "molecular condoms",' said Dr Polina Lishko, who led the study team at the University of California, Berkeley.
Sperm normally moves with a steady, rhythmic motion as they swim upstream in the reproductive track toward the egg (see BioNews 894). However, once sperm reach the egg, their tails switch to a drilling motion, allowing the head of the sperm to move through the egg's dense outer layers. Scientists call this manoeuvre the 'power kick'.
The key to this power kick is a calcium channel called CatSper, which is specific to sperm. When this is opened, calcium enters the sperm tail and triggers movement.
Last year, the researchers found that progesterone opens this channel by binding to a protein called ABHD2. They began to search for other chemicals – in plants used by indigenous people around the world - that would bind to this protein and either open or block the calcium ion channel.
The researchers found that lupeol and pristimerin blocked the hormone progesterone from binding to ABHD2, preventing the sperm’s power kick.
'These compounds not only blocked calcium channel activation, but also blocked sperm hyperactivated motility, reducing their activity to the level of nonactivated sperm cells,' Dr Lishko said. 'It is not toxic to sperm cells; they still can move. But they cannot develop this powerful stroke, because this whole activation pathway is shut down.'
'This is a very interesting study which shows that two natural compounds can knock out a key molecule on sperm that regulates how they swim in the final moments before fertilisation,' Professor Allan Pacey at the University of Sheffield, told the BBC. 'Moreover, because the molecule is specific to sperm, it seems a good bet that this could be a novel contraceptive target that might lead to a male contraceptive pill without any of the side-effects so far seen in trials with hormone-jab contraceptives.'
However, the cost of extracting these chemicals from the plants is high, as they are present at such low levels. A cheap source needs to be found if they are to be considered viable alternatives to current contraceptives.
The study is published in Proceedings of the National Academy of Sciences of the United States of America. The researchers are now testing how effective these chemicals are in preventing fertilisation of primate sperm and eggs in vitro.