IMMUNOHISTOCHEMICAL LOCALIZATION OF BILIVERDIN REDUCTASE IN RAT-BRAIN- AGE-RELATED EXPRESSION OF PROTEIN AND TRANSCRIPT

Biliverdin reductase regulates heme oxygenase activity by removing the inhibitory product of the oxygenase activity, biliverdin; and reducin g it to bilirubin. The other products of the oxygenase are carbon mono xide and Fe. To date, biliverdin reductase remains unique among all en zymes described by using 2 different cofactors (NADPH and NADH) at dif ferent pH ranges. The present study reports on the developmentally reg ulated changes in the pattern of protein expression and the level of b iliverdin reductase transcript in rat brain. Biliverdin reductase acti vity of the brain cytosol with both NADPH (pH 8.7) and NADH (pH 6.7) e xhibited developmental changes with the activity increasing after birt h, reaching an adult level by day 28 postpartum. When analyzed by West ern blotting the immunoreactive protein detected increased as the anim al matured (day 1 to 28 postparturition). Northern blot hybridization of RNA isolated from rat brain revealed the presence of similar to 1.5 kb biliverdin reductase transcript at all stages of development rangi ng from 1 day post partum to 20 months. The level of the transcript wa s developmentally regulated and a gradual increase (approximate to 4-f old) was observed from day 1 after birth to adulthood and was maintain ed in 20 month old animals. Cellular localization, using immunohistoch emical technique, revealed age-related pattern of expression of the re ductase in select regions such as the cortex, substantia nigra, hippoc ampus and in the cerebellum; the changes, however, did not follow the same pattern. To elaborate, in the cortex, the reductase expression in creased when 7-day-old animals were compared with young adults (2 mont hs old) and then declined in the 20-month-old animals. In the substant ia nigra the level of reductase expression progressively declined with age when 7-day-old neonate, 2- and 20-month-old animals were compared . In the hippocampus, a distinct reductase-expressing cell population residingbetween CA1 and the dentate gyrus was observed in the 7-day-ol d animals; these cells were not detected in the adults (2 or 20 months old). In the cerebellum, the expression of the reductase reflected th e developmental organization of this region. We postulate that age-dep endent increase of the brain reductase at the transcript and protein l evels in the course of maturation serves to control heme oxygenase act ivity which also displays a developmental pattern in the organ. As suc h, the reductase modulates generation of biologically active heme degr adation products; bilirubin, carbon monoxide and Fe.