alkaloid Any of the naturally occurring, basic nitrogen compounds (mostly heterocyclic) occurring mostly in the plant kingdom, but also found in bacteria, fungi, and animals. By extension, certain neutral compounds biogenetically related to basic alkaloids are also classed as alkaloids. Amino acids, peptides, proteins, nucleotides, nucleic acids, amino sugars and antibiotics are not normally regarded as alkaloids. Compounds in which the nitrogen is exocyclic (dopamine, mescaline, serotonin, etc.) are usually classed as amines rather than alkaloids. organonitrogen compound