Since a specific inhibition of cerebral spermidine (Spd) synthase activity by alicyclic amines was preliminarily observed in vitro, we examined the in vivo inhibitory effectiveness of dicyclohexylamine (DCHA) on Spd biosynthesis in 21-day-old rat brain. For this purpose a previously reported HPLC procedure (Porta et al., 1981a) was modified to analyze the cerebral levels of DCHA at the time of polyamine determinations. The intraperitoneally injected DCHA was shown to cross the blood-brain barrier easily, reaching high levels in the cerebral tissue (approximately 750 nmol/g brain) within 1 h of its administration. The effect of the drug on the polyamine metabolism resulted in a significant depletion of Spd biosynthesis from the sixth hour after the treatment and in an earlier and prolonged increase of the putrescine (Pt) steady-state levels. Conversely, the spermine (Spm) endogenous pools remained unchanged throughout the 24-h post-DCHA period. Moreover, following the intracerebral administration of [1,4-14C]Pt, significantly lower specific radioactivity (s.r.a.) values for labeled Pt and Spd were recorded in the brains of DCHA-treated animals. Conversely, after intracerebral [14C]Spd injection, the s.r.a. of newly formed [14C]Spm remained unchanged, confirming the specificity of the DCHA effect on the Spd biosynthesis.
Indolamine N-methyltransferase (INMT) has been purified to an apparent homogeneity from rabbit lung, and some of its catalytic and physicochemical properties have been examined. The enzyme is a monomeric protein with a molecular weight of 31,500 +/- 1000, a molecular Stokes radius of 21.5 A, and a diffusion coefficient of 8.7 X 10(-7) cm2/s. The frictional ratio of the native enzyme (1.05) suggests that the shape of the molecule is nearly spherical. Denaturation experiments performed with increasing concentrations of guanidine hydrochloride (Gdn-HCl) at neural pH indicated that the active site of the enzyme was destroyed by a structural rearrangement of the protein molecule without large change in its size and shape. The final state reached in 6.0 M Gdn . HCl seemed to correspond to a disulfide cross-linked randomly coiled polypeptide. Full normalization of the fluorescent parameter was attained only in the presence of 0.1 M beta-mercaptoethanol. A structural rearrangement has been observed upon acidification of INMT from pH 7.0 to pH 2.0. At pH 4.5, most of the peptide backbone appeared to be unorganized, but further acidification to pH 2.0 produced a reorganization of protein structure which became able to bind 8-anilino-1-naphthalenesulfonate. The data support the hypothesis that the enzyme structure results from the close package of organized regions joined by structureless segments.
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