ENPP1 and chronic kidney disease: Nevertheless, based on the aortic mRNA expression profiles of Npp1, Ntpd1 and Tnap, an increase in aortic PPi levels of vehicle- and TNAP inhibitor-treated CKD rats may reasonably be expected given the decrease in Pi-generating enzymes TNAP and NTPD1 and increase in PPi-generating enzymes Npp1 and Npp3. However, as mentioned above, it has been proposed that both Npp1 and Npp3 can break down PPi into Pi and, thus, can also act as Pi-generating enzymes.