A substance that, via chemical reactions, develops into a different substance. levodopa is a precursor for dopamine; enzymes facilitate a series of metabolic reactions that create the new chemical structure that is molecularly close to the precursor’s molecular structure. Similarly dopamine is a precursor for another neurotransmitter, nor-epinephrine, which is a precursor for epinephrine. All such conversions require interaction with other substances, however, which often are enzymes (protein-based structures), as this conversion is a key role of enzymes in the body.
A precursor also can be a circumstance or condition that subsequently becomes a disease. Insulin resistance from obesity is a precursor for type 2 diabetes, for example. A genetic defect such as a mutation in the alpha- synuclein gene may be a precursor for Parkinson’s disease, if there are events that occur to activate the gene’s defective functions and cause development of Parkinson’s.