Figure 1.

Schematic representation of nutritional intake and regulated biosynthesis to maintain polyamine homeostasis in the body. Nutritional intake into the blood stream includes arginine and orhithine in addition to the polyamines (PAs) putrescine, spermidine, and spermine. PAs are taken up mostly in the small intestine and delivered to the blood stream, but they cannot pass the blood–brain barrier. However, arginine and ornithine can and will represent starting materials of the regulated biosynthesis of PAs in the central nervous system. Inside the neuron, PA concentrations are tightly controlled by at least six proteins, represented as boxes in the bottom row: antizyme inhibitor, ornithine decarboxylase, S-adenosylmethionine decarboxylase, Spd/Spm-synthase, antizyme, and Spm/Spd-N¹-acetyltransferase. Arrows or blocked arrows indicate which target proteins (green: increase PAs; red: decrease PAs) are modulated by a given PA. The colour of the (x) indicates which PA is involved.