3-Hydroxybutyric acid (or beta-hydroxybutyrate) is a ketone body. Like the other ketone bodies (acetoacetate and acetone) levels of 3-hydroxybutyrate in blood and urine are raised in ketosis. In humans 3-hydroxybutyrate is synthesized in the liver from acetyl-CoA and can be used as an energy source by the brain when blood glucose is low.

Pyrrole-2-carboxylic acid was first identified as a degradation product of sialic acids then as a derivative of the oxidation of the D-hydroxyproline isomers by mammalian D-amino acid oxidase.

Cytosine is a pyrimidine base that is a fundamental unit of nucleic acids. The deamination of cytosine alone is apparent and the nucleotide of cytosine is the prime mutagenic nucleotide in leukaemia and cancer.