Among the three racemates of malic acid, L-malic acid has important physiological functions in the organism, and D-malic acid is non-physiological activity, so the biochemical metabolism of DL-malic acid is worse than that of L-malic acid. The living body must process D-malic acid with the help of the racemase secreted by the liver, and convert D-malic acid into L-malic acid by the oxidoreductase of the kidney before it can enter the metabolic cycle. Therefore, the application of DL-malic acid in food and medicine is limited, and infants and patients with liver and kidney diseases are not suitable for consumption. In 1970, the FDA stipulated that racemic DL-malic acid could not be used as an infant food additive.
L-malic acid is widely present in various organisms such as animals, plants and microorganisms. It is an important member of the basic oxidative metabolic cycle in the body - the tricarboxylic acid cycle (TCA), and is also an important member of the glyoxylic acid cycle. It is also the product of CO2 fixation reaction and is an essential nutrient absorbed by the human body. It is constantly updated in the metabolic process, sometimes used to synthesize other substances, sometimes derived from other substances, directly involved in the energy metabolism of mitochondria, and plays a pivotal role in the metabolic process of organisms. Therefore, L-malic acid plays an extremely important role in regulating metabolic activities in organisms.
The position of L-malic acid in the TCA cycle can be expressed as follows:
Fumaric acid+H2O—→L-malic acid + NAD+—→Oxaloacetic acid + NADH + H+
The enzymes involved in the reaction are mainly fumarase and malic enzyme.
In glyoxylic acid cycle:
Glyoxylic acid + Acetyl-CoA—→L-malic acid—→Oxaloacetic acid
In the CO2 fixation reaction, L-malic acid is is obtained by fixing CO2 with phosphoenolpyruvate or pyruvate to produce oxaloacetic acid, or directly generates L-malic acid from pyruvic acid through reductive carboxylation. L-malic acid produced by fixation of CO2 can also be oxidized and decarboxylated to pyruvate under the action of malic enzyme. L-malic acid plays a significant role in fighting fatigue, treating heart disease, protecting the liver, reducing the toxicity of drugs to kidney and bone marrow cells, and positively regulating learning and memory.
Therefore, the difference in the physiological functions between D-malic acid and L-malic acid leads to their different uses in various aspects of social industry.