Fig. 2
Synthesis of polyamines from proline and their function in the porcine placenta. Unlike most other tissues, which convert arginine to polyamines via arginase and ornithine decarboxylase, the porcine placenta lacks arginase activity, so it cannot synthesize ornithine from arginine. Degradation of ODC is regulated via OAZ, which binds to ODC, while AZIN, a protein with a similar structure to ODC but with no activity, can bind to OAZ with a higher affinity, thus preventing ODC degradation. Note that there are three isoforms of OAZ (OAZ1, OAZ2 and OAZ3) and two isoforms of AZIN (AZIN1 and AZIN2). DFMO is a catalytic, irreversible inhibitor of ODC. For porcine placentae, polyamines mainly have roles in angiogenesis, oxidative stress, protein synthesis, and apoptosis. Solid black arrows indicate direct activation, and dashed black arrows indicate indirect activation. ADC, arginine decarboxylase; α-KG, α-ketoglutarate; AZIN, antizyme inhibitor; DCAM, decarboxylated S-adenosylmethionine; DFMO, difluoromethylornithine; MTA, methylthioadenosine; mTOR, mechanistic target of rapamycin; OAT, ornithine aminotransferase; OAZ, antizyme; ODC, ornithine decarboxylase; P-5-C,△1-L-Pyrroline-5-Carboxylate; POX, proline oxidase; PPi, inorganic pyrophosphate; ROS, reactive oxygen species; SAM, S-adenosylmethionine; SAMD, S-adenosylmethionine decarboxylase; SAMS, S-adenosylmethionine synthase