The precursor for synthesis of spermine is the amino acid ornithine. It is a precursor to other polyamines, such as spermine and its structural isomer thermospermine. After that the putrescine follows the same pathway to completing the synthesis of spermine. For comparison the similar substance spermine, found in semen, is over 3 times as toxic. Its activity is inhibited by spermine, sphingosine, and several phospholipids. The polyamine spermine is produced, which triggers carpospore production. The principal examples are the triamine and tetraamine spermidine and spermine. Spermidine in turn is combined with another "S"-adenosylmethioninamine and gets converted to spermine. Similarly, a base such as spermine has more than one site where protonation can occur. Spermidine and spermine, kinds of polyamine, are found in reducing cadmium toxicity in rice. Spermine oxidase is an enzyme that in humans is encoded by the "SMOX" gene. Spermine is the chemical primarily responsible for the characteristic odor of semen. Spermine is a polyamine involved in cellular metabolism found in all eukaryotic cells.