Research Uncovers Why AIDS Can’t Be Transmitted Through Saliva

NEW YORK, NY -- March 16, 1999 -- NYU School of Medicine and NIH researchers have identified an ordinary protein present in tears and saliva as the long-sought mystery substance in the urine of pregnant women that is a powerful anti-HIV agent.

The new finding helps explain why AIDS cannot be transmitted through saliva and opens the way to an entirely new class of anti-AIDS medicines, according to a new study by the NYU and NIH scientists.

The researchers, led by Sylvia Lee-Huang, Ph.D., professor of biochemistry at NYU, and Hao-Chia Chen, Ph.D., research chemist, National Institute of Child Health and Human Development, found that lysozyme, a well-known protein abundant in tears, is also present in the urine of pregnant women and is a potent anti-HIV agent. They also identified other potent anti-HIV agents in the urine, proteins called ribonucleases, enzymes that break apart the genetic material of RNA viruses, including the AIDS virus.

"These proteins are very promising anti-AIDS agents and likely will be well tolerated by the body, causing few side effects, because they occur naturally," Dr. Lee-Huang said. "We look forward to the next phase of our work, which will further determine how these proteins can best be used against HIV and other viruses."

Lysozyme, an enzyme first described in 1922 by Alexander Fleming, the discoverer of penicillin, breaks down the cell wall of bacteria, causing bacterial cells to burst and die. The enzyme, which Fleming also discovered was abundant in tears, is produced in especially high levels during pregnancy and may play an important role protecting the body from foreign invaders like viruses and bacteria.

Although it isn't understood yet how the proteins actually inhibit HIV, Dr. Lee-Huang suspects that lysozyme and ribonuclease act together to inhibit the HIV virus. In this scenario, the lysozyme may be acting to break down the outer membrane of the virus, potentially interfering with its ability to invade immune cells. The ribonucleases may target viral RNA, possibly affecting viral replication.

The new study, published in today’s issue of The Proceedings of the National Academy of Sciences, caps an intensive five-year search for the powerful anti-AIDS and anti-cancer agent present in a most unusual source -- the urine of pregnant women. Researchers had suspected that human chorionic gonadotropin (hCG), a pregnancy hormone produced by the placenta, was the agent because it inhibited HIV-infected immune cells in the laboratory and in small clinical studies shrank Kaposi's sarcoma tumours in AIDS patients. Moreover, AIDS transmission rarely occurs during the first trimester of pregnancy when hCG levels are high. The main function of hCG is to stabilise pregnancy and protect the embryo from rejection by the mother's body.

However proof of hCG's effects wasn't straightforward. Although crude preparations of hCG inhibited the growth of Kaposi's sarcoma cells and HIV-infected cells in the laboratory, Dr. Lee-Huang and many other scientists showed that highly purified extracts of hCG failed to have any affect on these cells. So it wasn't clear whether hCG itself or some other protein in the urine from which it was prepared was responsible. Dr. Lee-Huang's laboratory narrowed the search to a portion of the hormone called the beta subunit; but to her surprise, the culprits turned out to be other proteins that are also present in the urine and stick to the beta subunit.

In the new study, Dr. Lee-Huang and co-workers isolated the beta subunit from commercial preparations of hCG prepared from the urine of pregnant women. Then, using techniques such as electrophoresis, liquid chromatography and amino acid sequencing, they painstakingly separated and purified the proteins present in the subunit and tested each against HIV-infected cells, as well as analysed their amino acid sequences. The entire process took about two years.

The amino acid sequences of the newly identified anti-viral components associated with the beta core fragment of the beta subunit of hCG are identical to those of human lysozyme and ribonucleases. Why these compounds cling tightly to the pregnancy hormone is a fascinating question, Dr. Lee-Huang said.

"Here again, it seems that Mother Nature knows best how to protect the earliest stages of human life," she explained.

After the lysozyme and ribonuclease were identified, the researchers tested lysozymes found in mother's milk, white blood cells and chicken egg white as well as ribonuclease found in the pancreas of cows. All of these proteins inhibited HIV and were incredibly potent.