Dihydropteridine Reductase from Man and from the Rat

Abstract

A reproducible purification procedure has been used to purify dihydropteridine reductase (DHPR) from human brain. The Kinetic constants and M.Wt. were determined for the purified enzyme. Purified DHPR was found to be inhibited in the presence of phenylpyruvate, 6-hydroxydopamine, I-methyl-4-phenyl-l, 2, 5, 6-tetrahydropyridine (MPTP) and aluminium. The in vivo effect of the neurotoxins lead and aluminium on DHPR has been studied. Human blood DHPR activity was reduced in lead workers and haemodialysis patients with chronic renal failure. Animal experiments showed that a brief hyperphenylalaninaemia in healthy rats following an administration of phenylalanine or para-chlorophenylalanine prior to a load of phenylalanine, produces a decrease in rat brain BH4% and actual BH4 levels compared to controls. In rats, the oestrogen diethylstilboestrol has been found to significantly decrease brain total biopterin, actual BH4 and BH4% level compared to matched controls dosed with corn oil only. Tetrahydrobiopterin metabolism is disrupted in a number of disease states. DHPR activity in the brain of patients dying from senile dementia and Down's syndrome, has been found to be higher than controls. The kinetic studies showed that Km values are significantly higher in the demented patients as correlated with controls. Breast neoplastic tissue showed a significant increase in DHPR activity in the basis of protein and DNA level, as compared to apparently normal tissue from the same breast. A single case of partial heterozygote DHPR deficiency was investigated. Whole blood DHPR activity was less than half that of normal subjects. The kinetic studies showed significantly increased Km values for this case in comparison to normals. The mechanism and consequences of such change in tetrahydrobiopterin metabolism are discussed in the light of in vivo studies as well as the in vitro results presented in this thesis.