An In-Silico Investigation of Menthol Metabolism

Abstract

6Prevalence of mentholated products for consumption has brought great importance to studies on menthol's 7 metabolic pathways to ensure safety, design more potent derivatives, and identify therapeutic benefits. Proposed 8 pathways of (-)-menthol metabolism based on metabolites found experimentally in previous works by Yamaguchi, 9 Caldwell & Farmer, Madyastha & Srivatsan and Hiki et al. were not in agreement. This in silico approach is based 10 on the three in vivo studies and aims to resolve the discrepancies. Reactions in the pathways are conjugation with 11 glucuronic acid/sulfate, oxidation to alcohol, aldehyde & carboxylic acid, and formation of a four-membered/five-12 membered ring. Gas-phase structures, standard Gibbs energies and SMD solvation energies at B3LYP/6-13 311++G(d,p) level were obtained for 102 compounds in the pathways. This study provides a more complete 14 picture of menthol metabolism by combining information from three experimental studies and filling missing links in 15 previously published pathways. 16 Introduction 17 (-)-Menthol or 1S,3R,4S-menthol is a naturally occurring compound found in plants of the Mentha genus 18 commonly known as mint. It is the most abundant in nature among the 8 possible stereoisomers, and make up at 19 least 50% of peppermint (Mentha piperita) oil and 70-80% of corn mint (Mentha arvensis) oil [1]. (-)-Menthol, 20 commonly referred to as menthol, has characteristic minty smell and flavor and exerts a cooling sensation when 21 applied to the skin and mucosal membranes [2]. Other isomers differ slightly in odor and physical characteristics 22 and do not possess the cooling action [3, 4]. 23 Menthol finds a wide range of applications from personal care products, medications, and confectionery to 24 pesticides and cigarettes. The popularity of the compound as a flavoring agent ranks third most important after 25 vanilla and citrus [5], and the annual production of menthol in India alone is in excess of 200 thousand metric tons 26 [6]. Mentholated products can be purchased as prescribed or over-the-counter medications as alleviators of 27 common cold and respiratory conditions [7], inhibitors of growth of foodborne pathogens [8], and analgesics [9].28 Considering its wide range of applications, mechanisms of action of menthol were relatively unknown until 29 recently. The cooling sensation is a result of the activation of transient receptor potential melastatin-8 (TRPM8), 30 an ion channel selective to temperature, voltage, and menthol [10]. Experimental evidence also show that (-)-31 menthol can selectively activate κ -opioid receptors in mice and, as a result, leads to its analgesic properties [9].are shown on the right. Lists of compounds and reactions are provided in Table 1 and Table 2, respectively. 62 63This in silico investigation is based on the metabolites identified experimentally by the three in vivo studies [17][18][19]. 64 We aim to resolve discrepancies and missing links found in these three studies by proposing more complete 65 pathways in F...