Aldehyde dehydrogenase from adult human brain that dehydrogenates γ-aminobutyraldehyde: purification, characterization, cloning and distribution

Abstract

Enzyme purification and characterization, cDNA cloning and Northern blot analysis were the techniques utilized during this investigation to determine the identity and occurrence of the aldehyde dehydrogenase that metabolizes γ-aminobutyraldehyde in adult human brain. The purification yielded one major protein which was active with γ-aminobutyraldehyde. It had the physicochemical and kinetic properties of the human liver E3 isoenzyme of aldehyde dehydrogenase (EC 1.2.1.3), and also interacted with an anti-(liver E3 isoenzyme) antibody. Tryptic peptides derived from the purified brain protein matched the amino acid sequence of the liver E3 isoenzyme. Employing liver E3 cDNA, a human cerebellar cDNA library was screened and a 2.0 kb cDNA fragment was isolated. The cerebellar cDNA yielded a derived primary structure which differed from the liver E3 amino acid sequence by a single serine-to-cysteine substitution at position 88 (position 84 in the liver sequence). Thus the γ-aminobutyraldehyde-metabolizing enzyme from human brain can be identified as E3′, a variant of the E3 isoenzyme. The catalytic properties of the brain variant were indistinguishable from those of E3, and so the functional importance of this variant is at present unknown. The distribution of this enzyme in brain was investigated by Northern blot analysis, which demonstrated the presence of E3′ mRNA in all regions of the human brain. mRNA levels were variable in the different brain areas, with the highest levels in the spinal cord and the lowest in the occipital pole.