This paper summarizes the results of my quarter-century study on the regulatory mechanism of ornithine decarboxylase (ODC), a key enzyme in polyamine synthesis. ODC turns over rapidly and is under negative feedback regulation by polyamines, which accelerate ODC degradation and induce an ODC-inhibitory protein, antizyme. Rat-liver ODC levels exhibit a circadian rhythm with the peak at night, owing to the feeding pattern. The dietary induction of hepatic ODC depends on both the quantity and quality of dietary proteins. In order to clarify the molecular mechanism of ODC regulation, rat-liver ODC was purified to homogeneity. Antizyme was also highly purified and its cDNAs were obtained. Forced expression of antizyme caused rapid ODC degradation, proving that antizyme mediates polyamine-induced destabilization of ODC. In newly established in vitro ODC degradation systems, ODC is degraded in an ATP- and antizymedependent manner by 26 S proteasome without ubiquitination. Antizyme also inactivates polyamine uptake. Nucleotide sequence analysis of antizyme mRNA revealed that programmed ribosomal frameshifting evoked by polyamines is necessary for its translation.
Effects of dietary phosphate levels and phosphate sources on iron utilization were studied. Seventy-two Wistar-strain male rats (70g body weight) were divided into 12 groups. Each group was fed one of four phosphate sources, either KH2PO4, K5P3O10, NaH2PO4 or Na5P3O10, and the diet was supplemented to approximately 0.5%, 1.0% or 1.5% P. Results: 1) The high dietary phosphate level (1.5%) resulted in significant decreases in weight gain, food intake and nitrogen retention. 2) Absorption and retention of Fe decreased, and urinary excretion increase, with increasing P intake by K2P3O10 and Na5P3O10 feeding.