There is an evidence of a direct in vitro inhibitory effect of HICA on various matrix metalloproteinase enzymes, which are responsible for degradation of various connective and protein tissues [14]. The delayed onset of muscle soreness (DOMS) is the sensation of muscular discomfort and pain during active contractions that occurs in a delayed fashion after strenuous exercise. Subjects with DOMS have painful, tender, and swollen muscles with reduced range of motion of adjacent joints especially after unaccustomed exercise [16, 17]. In addition to muscle tenderness with palpation, prolonged strength loss and a reduced range of motion are observed. These symptoms develop 24 to 48 hours after exercise,

and they disappear within 5 to 7 days [16, 17]. The pathophysiology of DOMS remains still undetermined, but it has been reported that after strenuous exercise muscle cell damage and inflammatory CH5424802 mw cells are observed see more in damaged muscle [16, 17]. Although leucine has a unique role as a promoter of protein synthesis [18], maybe especially the metabolites of leucine decrease

breakdown of proteins, particularly muscle proteins [11]. The roles and mechanisms of actions of leucine and its metabolites are not clear and even confusing. For instance, α-ketoisocaproate (KIC), derived from leucine by transamination, is anti-catabolic and reduces muscle protein degradation when given as intravenous infusion [11]. On the other hand, it is a potent inhibitor of branched-chain α-keto acid dehydrogenase kinase and may lead to increased catabolism of branched chain amino acids (BCAAs) [19]. β-Hydroxy β-methylbutyric acid (HMB) or β-hydroxy β-methylbutyrate is another metabolite of SPTBN5 leucine and plays also a role in protein synthesis and breakdown [20]. Recently [21], it was observed that 14 of HMB and KIC supplementation reduced signs and symptoms of exercise-induced muscle damage in non-resistance trained males following a single bout of resistance exercise emphasizing eccentric contractions. There are separate mechanisms to control protein synthesis and proteolysis [22].

Tischler et al [11] suggested that the first step in controlling muscle proteolysis by leucine is the oxidation of leucine, catalyzed by aminotransferase enzyme. The end product of the reaction is keto leucine (α-ketoisocaproate, KIC) but, in certain situations, it can be HICA as well. It is suggested that the aminotransferase enzyme is responsible to oxidize leucine both to its keto (KIC) and to its hydroxyl form (HICA) and both reactions are reversible [23]. The reaction between keto and hydroxyl leucine is an equilibrium reaction with oxidoreduction equilibrium constant (thermodynamic constant) Keq = 3.1 ± 0.2 × 10-12 mol/l and the reaction half time is 230 min towards oxygenation in human. Keto acid is irreversibly oxidized by mitochondrial ketoacid dehydrogenaze [24]. Irreversible degradation of keto acids is higher in liver than that in muscle [24].