Down-regulation of aminolevulinate synthase, the rate-limiting enzyme for heme biosynthesis in Alzheimer's disease.

Heme is an essential cell metabolite, intracellular regulatory molecule, and protein prosthetic group. Given the known alterations in heme metabolism and redox metal distribution and the up-regulation of heme oxygenase enzyme in Alzheimer's disease (AD), we hypothesized that heme dyshomeostasis plays a key role in the pathogenesis. To begin testing this hypothesis, we used qRT-PCR to quantify the expression of aminolevulinate synthase (ALAS1) and porphobilinogen deaminase (PBGD), rate-limiting enzymes in the heme biosynthesis pathway. The relative expression of ALAS1 mRNA, the first and rate-limiting enzyme for heme biosynthesis under normal physiological conditions, was significantly (p<0.05) reduced by nearly 90% in AD compared to control. Coordinately, the relative expression of PBGD mRNA, which encodes porphobilinogen deaminase, the third enzyme in the heme synthesis pathway and a secondary rate-limiting enzyme in heme biosynthesis, was also significantly (p<0.02) reduced by nearly 60% in AD brain compared to control and significantly related to apolipoprotein E genotype (p<0.005). In contrast, the relative expression of ALAD mRNA, which encodes aminolevulinate dehydratase, the second and a non-rate-limiting enzyme for heme biosynthesis, was unchanged between the two groups. Taken together, our results suggest regulation of cerebral heme biosynthesis is profoundly altered in AD and may contribute toward disease pathogenesis by affecting cell metabolism as well as iron homeostasis.