D-amino acids are unusual because the predominant form for amino acids in humans and mammals are the L-form. However, a number of D-form amino acids are found. In particular, D-aspartic acid is an amino acid that may play a central role in mammalian metabolism and this may include humans. In rat experiments, supplements of D-aspartate are able to stimulate testosterone production. D-aspartate is found in high concentrations in the testes, adrenal gland and pituitary gland, indicating it may play an important role in metabolism. Synthesis of testosterone is associated with uptake of D-aspartate into the testes, and when this process is inhibited, both D-aspartate levels in the testes, and testosterone synthesis, declines. One suggestion is that D-aspartate stimulates the uptake of cholesterol to the testes, and as this step is the rate limiting step in testosterone synthesis, the reaction proceeds at a faster rate. This may also explain the increase in testosterone seen with administration of D-aspartate to humans.
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