The nature and distribution of PPT-1 has been studied in the human basal ganglia. The protein is expressed evenly throughout the caudate and putamen, and 80 to 85% of it exists in the beta-PPT isoform. 15-20% of the protein is in the gamma-PPT isoform, while no alpha-PPT was detected at all.
In humans, beta-PPT is the dominant isoform in the brain, which contrasts with rats (predominantly gamma-PPT) and cows (alpha-PPT).
While both human and rat PPT-1 produce substance P and neurokinin A, humans produce more neuropeptide K, whereas rats produce more neuropeptide gamma. In cow brains, PPT-1 primarily encodes substance P, but not other neurokinin A-derived peptides.
^Chiwakata C, Brackmann B, Hunt N, Davidoff M, Schulze W, Ivell R (May 1991). "Tachykinin (substance-P) gene expression in Leydig cells of the human and mouse testis". Endocrinology128 (5): 2441–8. doi:10.1210/endo-128-5-2441. PMID1708336.
Harmar AJ, Armstrong A, Pascall JC, et al. (1986). "cDNA sequence of human beta-preprotachykinin, the common precursor to substance P and neurokinin A.". FEBS Lett.208 (1): 67–72. doi:10.1016/0014-5793(86)81534-4. PMID3770210.