Endogenous sources of formaldehyde and acetaldehyde. The spontaneous chemical decomposition of tetrahydrofolate and dihydrofolate releases formaldehyde. Mitochondrial enzymes dimethylglycine dehydrogenase (DMGDH) and sarcosine dehydrogenase (SARDH) release formaldehyde as catalytic byproducts. Cytochrome P450 (CYP450) enzymes located in the mitochondria and endoplasmic reticulum can generate formaldehyde through the oxidation of glycerol. Oxidative demethylases release formaldehyde through cleavage of methyl groups found on histone proteins, DNA, and RNA. Polyunsaturated lipid species can be oxidized by reactive oxygen species (ROS) in a process called lipid peroxidation to release a range of aldehydes including formaldehyde and acetaldehyde. In myeloid cells such as neutrophils and monocytes, myeloperoxidase enzyme found in the lysosomes and secreted into the surrounding extracellular environment can generate hypochlorous acid (HOCl), which can oxidize the amino acids glycine and alanine to release formaldehyde and acetaldehyde, respectively. Outside of the cell, the oxidative deamination of methylamine and aminoacetone by serum amine oxidase can release formaldehyde and acetaldehyde, respectively. Ingested methanol and ethanol can be enzymatically converted to formaldehyde and acetaldehyde by alcohol dehydrogenase enzymes expressed mainly in the liver (ADH).