Green-Plant Enzyme Puts a Red Light on Porphyrins

Porphyrins turn light into life. Plants need them for photosynthesis. Animals, including humans, use them for transporting oxygen in their blood. But excess porphyrin is harmful. Now USDA scientists, working with medical researchers at Dartmouth, have discovered how some green plants may regulate these life-giving chemicals--a finding that could someday protect crops or people.

The scientists found a natural plant enzyme which deactivates porphyrins in plants and animals. The concept came from cooperative research between Dartmouth Medical School and ARS' Natural Products Utilization Research Unit in Oxford, Miss., on the campus of the University of Mississippi.

In green plants, chlorophylls, which are derived from porphyrins, trap solar energy and turn it into food. Some herbicides prevent chlorophyll synthesis and cause porphyrins to accumulate to abnormally high levels. Plants then become hypersensitive to light and die.

This works well for killing weeds, but since the herbicide is applied when crops are in the field they, too, could be injured. Crops could be bred, or genetically engineered, to enhance their levels of the enzyme that deactivates the porphyrins. This would protect the crops when the herbicide is applied.

This discovery in plants may also lead to new ways of preventing or controlling porphyria, a human genetic disease. Cells of people with this condition don't properly convert porphyrins into heme, the deep-red, iron-rich component of hemoglobin. Porphyrin buildup can cause nerve malfunction, resulting in weakness, nausea and skin rash; and gives urine a port-wine color. People affected with this condition also become hypersensitive to light.

Scientific contact: Franck E. Dayan, ARS National Center for the Development of Natural Products, Oxford, Miss., phone (601) 232-1039, fdayan@ag.gov.