The Absolute Optical Specificity of Monoamine Oxidase

104

The application of positron emission tomography (PET) to the study of biochemical transformations in the living human and animal body requires the development of highly selective radiotracers whose concentrations in tissue provide a record of a discrete metabolic process. L-N-[11C-methyl]Deprenyl ([11C]L-deprenyl), a suicide inactivator of monoamine oxidase (MAO) type B, has been developed as a radiotracer for mapping MAO B in the living human and animal brain. In this investigation, [11C]L-deprenyl (1) and [11C]L-deprenyl-alpha, alpha-2H2 (2) have been compared in three different baboons by PET measurement of carbon-11 uptake and retention in the brain and the measurement of the amount of unchanged tracer in the arterial plasma over a 90-min time interval. For one baboon, N-[11C-methyl-2H3]L-deprenyl (3) was also studied. Kinetic parameters calculated using a three-compartment model revealed a deuterium isotope effect of 3.8 +/- 1.1. Comparison of the two tracers (1 and 2) in mouse brain demonstrated that deuterium substitution significantly reduced the amount of radioactivity bound to protein. HPLC and GLC analysis of the soluble radioactivity in mouse brain after injection of [11C]L-deprenyl showed the presence of [11C]methamphetamine as a major product along with unidentified labeled products. Sodium dodecyl sulfate-polyacrylamide electrophoresis with carbon-14-labeled L-deprenyl showed that a protein of molecular weight 58,000 was labeled. These results establish that MAO-catalyzed cleavage of the alpha carbon-hydrogen bond on the propargyl group is the rate limiting (or a major rate contributing) step in the retention of carbon-11 in brain and that the in vivo detection of labeled products in brain after the injection of [11C]L-deprenyl provides a record of MAO activity.

Section: Discussionmentioning

confidence: 99%

Mechanistic Positron Emission Tomography Studies: Demonstration of a Deuterium Isotope Effect in the Monoamine Oxidase‐Catalyzed Binding of [¹¹C]l‐Deprenyl in Living Baboon Brain

Fowler

Wolf

MacGregor

et al. 1988

104

“…a,a-Deuteration of PE retards its degradation by M A 0 in vitro (Yu et al, 1981). In fact, in the deamination of C Y ,~-[~H~] P E by M A 0 only the (R)-a-C-['H] bond and not the (S)-CY-C-[~H] bond exerts an isotope effect (Belleau et al, 1960;P. H. Yu, B.…”

Section: Discussionmentioning

confidence: 99%

Effects of Deuterium Substitution on the Catabolism of β‐Phenylethylamine: An In Vivo Study

Dyck

Burden

Boulton

1986

beta-Phenylethylamine (PE) hydrochloride injected intraperitoneally into rats was distributed evenly throughout the various regions of rat brain. Similarly, when a mixture of PE and alpha, alpha, beta, beta-deuterated PE [( 2H4]PE) was injected, no regional differences were observed in the ratios of the amounts of [2H4]PE and PE present; however, significantly more [2H4]PE than PE was present, although a 1:1 mixture had been administered. Further experiments in which the amounts of [2H4]PE and PE in whole rat brain, liver, and plasma were quantified confirmed this finding. The maximum [2H4]PE-to-PE ratios observed were 67 in whole brain 1 h after injection and 8 in liver and in plasma 45 min after injection. The whole brain [2H4]PE-to-PE ratios were decreased by pargyline pretreatment. Subsequent experiments showed that more alpha, alpha-[2H2]PE than PE was present in whole brain, liver, and plasma of rats injected with an equimolar mixture of alpha, alpha-[2H2]PE and PE. In contrast, beta, beta-[2H2]PE was not enriched in comparison to PE under the same experimental conditions. We concluded that the basis for the enrichment of [2H4]PE and alpha, alpha-[2H2]PE compared to PE was due to protection of the deuterated analogs from the actions of monoamine oxidase and perhaps aldehyde dehydrogenase; this protection led to pronounced deuterium substitution effects in vivo especially in the brain.

“…A DIE only takes place, therefore, when deuterium is substituted at the y-carbon position. Because DIEs often occur in nonenzymatic reactions, for example, in the ninhydrin reaction with primary amines used previously to assess DIEs in monoamine oxidation (Belleau et al, 1960), it was necessary to assess whether the high DIE found in the present experiment could be partly due to such an effect occurring in the condensation reaction between the deuterated SSA product and cyclohexane-1,3-dione to produce a fluorophore. This can be ruled out, because the radiometric and mass spectrometry procedures yielded DIES of similar magnitude.…”

Section: Kinetic Parameters Of the Die In Gaba Transaminationmentioning

confidence: 99%

Deuterium Isotope Effect in γ‐Aminobutyric Acid Transamination: Determination of Rate‐Limiting Step

Durden

Davis

et al. 1987

The rate of transamination of gamma-aminobutryic acid (GABA) catalyzed by hog brain gamma-aminobutyrate aminotransferase was substantially reduced when the hydrogen at the gamma-carbon position was replaced by deuterium. The deuterium isotope effect of this reaction has been substantiated by fluorometric, radiometric, and mass spectrometric procedures and assessed kinetically. The ratios of Vmax of the nonlabeled substrate/Vmax of the deuterated substrate obtained under different conditions ranged from 6 to 7. This indicates that the cleavage of the hydrogen from the gamma-carbon is the rate-determining step in GABA transamination. Similar isotope effects have also been shown to occur in the peripheral system in vivo.