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Japanese researchers make breakthrough in antioxidant enzyme linked to jaundice

Japanese researchers discover how biliverdin reductase enzyme produces bilirubin, responsible for yellow color of bile and jaundiced skin

Osaka University


IMAGE: Bilirubin is the causative material of jaundice. view more

Credit: Kei Wada


Now, a Japanese research collaboration involving Osaka University believes it has the answer. BR is already known to be produced from a related chemical, biliverdin (BV), by the enzyme biliverdin reductase (BVR). The enzyme wraps around BV and transfers two hydrogen atoms - one positive and one negative - to produce the yellow antioxidant. However, biologists could not establish which part of the enzyme was chemically involved in the process (the active site), or where the positive hydrogen came from. The findings were recently reported in Nature Communications.

"Previous studies used BVR from rats, and could never crystallize the enzyme well enough to determine how it binds to BV," study co-author Keiichi Fukuyama says. "We realized that the same enzyme in Synechocystis bacteria had an almost identical fold-shape, but was easier to examine by X-ray crystallography."

To their surprise, the researchers found two molecules of BV - one stacked upon the other - at the active site, even though only one is converted to BR. From the X-ray data, they deduced why two were needed. The first BV, because of the direction it faces to receive the negative hydrogen, cannot accept the positive hydrogen directly from the enzyme. Instead, the second BV acts as a hydrogen conduit to enable the process, but is not itself converted to BR.


"Such neat stacking of two substrate molecules during enzymatic processes is very rare," Fukuyama says. "A better understanding of BR production is not just exciting for biochemistry, but should help us develop drugs for diseases marked by severe jaundice from excess BR, such as hepatitis."

