Methylxanthines Inhibit Primary Amine Oxidase and Monoamine Oxidase Activities of Human Adipose Tissue

Abstract

Background: Methylxanthines including caffeine and theobromine are widely consumed compounds and were recently shown to interact with bovine copper-containing amine oxidase. To the best of our knowledge, no direct demonstration of any interplay between these phytochemicals and human primary amine oxidase (PrAO) has been reported to date. We took advantage of the coexistence of PrAO and monoamine oxidase (MAO) activities in human subcutaneous adipose tissue (hScAT) to test the interaction between several methylxanthines and these enzymes, which are involved in many key pathophysiological processes. Methods: Benzylamine, methylamine, and tyramine were used as substrates for PrAO and MAO in homogenates of subcutaneous adipose depots obtained from overweight women undergoing plastic surgery. Methylxanthines were tested as substrates or inhibitors by fluorimetric determination of hydrogen peroxide, an end-product of amine oxidation. Results: Semicarbazide-sensitive PrAO activity was inhibited by theobromine, caffeine, and isobutylmethylxanthine (IBMX) while theophylline, paraxanthine, and 7-methylxanthine had little effect. Theobromine inhibited PrAO activity by 54% at 2.5 mM. Overall, the relationship between methylxanthine structure and the degree of inhibition was similar to that seen with bovine PrAO, although higher concentrations (mM) were required for inhibition. Theobromine also inhibited oxidation of tyramine by MAO, at the limits of its solubility in a DMSO vehicle. At doses higher than 12 % v/v, DMSO impaired MAO activity. MAO was also inhibited by millimolar doses of IBMX, caffeine and by other methylxanthines to a lesser extent. Conclusions: This preclinical study extrapolates previous findings with bovine PrAO to human tissues. Given that PrAO is a potential target for anti-inflammatory drugs, it indicates that alongside phosphodiesterase inhibition and adenosine receptor antagonism, PrAO and MAO inhibition could contribute to the health benefits of methylxanthines, especially their anti-inflammatory effects.

Novel Facet of an Old Dietary Molecule? Direct Influence of Caffeine on Glucose and Biogenic Amine Handling by Human Adipocytes

“…Benzylamine oxidation by human adipose tissue preparations is totally abolished by semicarbazide inhibition as already reported [37], therefore due solely to SSAO/PrAO, and was impaired in the pres-ence of caffeine (Figure 7). This was in agreement with previous observations of caffeine inhibition of human PrAO [27]. In the concentration range 0.1-2.5 mM, caffeine inhibited human PrAO with a broadly mixed pattern of inhibition.…”

“…Human adipose tissue readily oxidized tyramine and benzylamine as recently reported [27]. The inhibition of human MAO by caffeine, already described for recombinant enzymes by Petzer et al [24], was confirmed in human adipose tissue preparations: Ki was 0.33 mM and Ki' was 1.8 mM (mixed inhibition; data not shown).…”

“…This prompted us to hypothesize that, aside from their lipolytic effect, methylxanthines influence glucose transport and incorporation into lipids (known as de novo lipogenesis) independently from their actions on cAMP levels, and differently from other lipolytic agents. Since we previously observed that the PrAO/MAO non selective inhibitor phenelzine is antilipogenic [21] and considering that methylxanthines inhibit both PrAO and MAO [18,[25][26][27], our attention was directed to investigate whether PrAO inhibition was a mechanism necessary and sufficient for caffeine inhibition of glucose metabolism, as suggested in a recent review by Papukashvili et al [28]. Three other findings supported this assumption: (1) PrAO substrates, such as benzylamine and methylamine stimulate glucose transport in human adipocytes [37,39]; (2) another phytochemical, resveratrol, is able to inhibit both MAO and lipogenic activities in mouse fat cells [40]; (3) as for caffeine, pharmacological inhibitors of SSAO have been proposed as potential anti-obesity agents [23].…”

“…First of all, the observed inhibition of glucose metabolism and of amine oxidase activity reported here for caffeine could not be considered as artefactual. Since caffeine was readily soluble in water at 10 mM, the use of vehicles such as DMSO or ethanol to dissolve the substance was not necessary, as it was the case for other naturally occurring methylxanthines, for which vehicle effects needed corrections and were far from negligible [27].…”

“…We have reported that several inhibitors of PrAO, such as phenelzine, inhibit lipogenesis in rodent adipocytes [21,22], and proposed them as potential antiobesity agents [23]. In addition, caffeine has been observed to inhibit both MAO [24] and PrAO [25,26], and recently these observations were extended to the amine oxidases expressed in human adipose tissue [27]. It was therefore tempting to hypothesize that inhibition of PrAO could contribute to the antilipogenic effect of caffeine.…”