MOLECULE

Malic acid

Malic acid is odorless and a tart-tasting organic dicarboxylic acid that plays a role in many sour or tart foods. Apples contain malic acid, which contributes to the sourness of a green apple. Indeed, malic acid was first isolated from apple juice by Carl Wilhelm Scheele in 1785. Malic acid can make a wine taste tart, although the amount decreases with increasing fruit ripeness. In its ionized form, malic acid is called malate. Malate is an intermediate of the TCA cycle along with fumarate. It can also be formed from pyruvate as one of the anaplerotic reactions. Malic acid is an extremely weak acidic compound (based on its pKa). Malic acid exists in all living organisms, ranging from bacteria to humans. In humans, malic acid is both derived from food sources and synthesized in the body through the citric acid cycle or Krebs cycle which takes place in the mitochondria. Malate's importance to the production of energy in the body during both aerobic and anaerobic conditions is well established. Under aerobic conditions, the oxidation of malate to oxaloacetate provides reducing equivalents to the mitochondria through the malate-aspartate redox shuttle. During anaerobic conditions, where a buildup of excess reducing equivalents inhibits glycolysis, malic acid's simultaneous reduction to succinate and oxidation to oxaloacetate is capable of removing the accumulating reducing equivalents. This allows malic acid to reverse hypoxia's inhibition of glycolysis and energy production. In studies on rats, it has been found that only tissue malate is depleted following exhaustive physical activity. Other key metabolites from the citric acid cycle needed for energy production were found to be unchanged. Because of this, a deficiency of malic acid has been hypothesized to be a major cause of physical exhaustion. Notably, the administration of malic acid to rats has been shown to elevate mitochondrial malate and increase mitochondrial respiration and energy production. Malic acid is abundant in many fruits and is the main acid found in apricots, blackberries, blueberries, cherries, mirabelles, peaches, pears, plums, and quince. Malic acid is also present in grapes and in most wines with concentrations as high as 5 g/l. The taste of malic acid is very clear and pure in rhubarb, a plant for which it is the primary flavor. It is also a component of some artificial vinegar flavors, such as "salt and vinegar" flavored potato chips. Therefore malic acid is a potential biomarker for the consumption of these foods.