2-Oxo-3-phenylpropanoic acid (Phenylpyruvic acid) is used in the synthesis of 3-phenyllactic acid (PLA) by lactate dehydrogenase.

All trans-Retinal (Retinaldehyde, Vitamin A aldehyde, Retinene), a component of the retinoid cycle, is converted to retinoic acid in vivo by the action of retinal dehydrogenase. Retinoic acid is a ligand for both RAR and RXR. All trans-retinal ias a potent photosensitizer.

H-Gly-Pro-OH (Glycyl proline, Glycyl-L-proline) is a dipeptide composed of glycine and proline, and is an end product of collagen metabolism that is further cleaved by prolidase. The resulting proline molecules are recycled into collagen or other proteins.

3-Aminoisobutyric acid (3-Amino-2-methylpropanoic acid, 3-Aminoisobutyrate, β-aminoisobutyric acid) is a product formed by the catabolism of thymine.

α-bisabolol is a small, plant-derived, oily sesquiterpene alcohol with some anti-inflammatory and even anti-microbial properties.

4-Guanidinobutanoate (4-Guanidinobutanoate, gamma-Guanidinobutyric acid, gamma-Guanidinobutanoate) is a normal metabolite present in low concentrations.

5-Methylcytidine is a modified nucleoside derived from 5-methylcytosine. It is found in ribonucleic acids of animal, plant, and bacterial origin.

Piperonyl acetone is a food additive used in food flavouring.

16-Dehydroprogesterone is a chemical derived from a progesterone.

Anserine (β-alanyl-N-methylhistidine) is a dipeptide containing β-alanine and 1-methylhistidine. It is normally absent from human tissues and body fluids, and its appearance there is an artifact of diet and serum carnosinase deficiency.