Strain-specific differences in levels of the mRNA for the epinephrine synthesizing enzyme phenylethanolamine N-methyltransferase.

The activity of the epinephrine biosynthetic enzyme phenylethanolamine N-methyltransferase (PNMT, EC 2.1.1.28) is 3- to 8-fold greater in rats of the Fischer 344 and Buffalo strains. The biochemical basis for the strain differences has been analyzed at the level of PNMT protein and messenger RNA production. Fischer rat adrenals possess approximately 5-fold more PNMT protein than those of the Buffalo rat as established by Western blotting and immunoprecipitation of adrenal gland homogenates. Poly(A)+ RNAs purified from adrenal glands of each strain were translated in a reticulocyte lysate system, immunoprecipitated with antibody to PNMT and fractionated by SDS-PAGE. A 35S-labelled protein of Mr = 34,000 was immunoprecipitated from adrenals of Fischer and Buffalo rats, indicating that the molecular weights of PNMT do not differ in these strains prior to post-translational processing. Hybridization of a 740 base pair (bp) cDNA for PNMT indicated that the mRNAs for PNMT are the same size in the adrenals of both strains. However, the adrenals of Fischer rats contain 2- to 4-fold more PNMT mRNA than Buffalo rats, as established by quantitative dot blot hybridization and Northern blot analysis. The medulla oblongata, the site of cell bodies of central adrenergic neurons, also contains approximately 2-fold more PNMT mRNA in Fischer rats. The strain specificity in the production of PNMT reflects differences in the expression of the gene for PNMT. Thus, an inherited capacity for PNMT expression may in fact provide the intrinsic determinants responsible for neurotransmitter production. These data provide a direct link between regulation of catecholamine enzyme biosynthesis at the genomic level and the availability of specific catecholamines for neurotransmitter and hormonal functions.