Synthesis of Receptor Sites for Vitamin B_<12> in Euglena gracilis

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In Euglena gracilis, vitamin B_<12> uptake follows a biphasic pattern consisting of an initial rapid phase followed by a slower secondary phase. Chase experiments showed that vitamin B_<12> was tightly bound to its receptor-sites during either rapid or slow phase of uptake. The slow phase was markedly inhibited when Euglena cells were preincubated with cycloheximide for 30 min at a concentration of 100 μg/ml. When the preincubation time was longer than 30 min, a gradual inhibition of the rapid phase occurred and reached 84% after 4 h. This inhibitory effect of cycloheximide is reversible. On the other hand, tunicamycin at 1 μg/ml irreversibly inhibited the B_<12> uptake suggesting that the receptor-site for B_<12> is glycoprotein in nature. These results suggest that the rapid phase is also dependent on protein synthesis and represents B_<12> binding on preexisting free receptors whereas the secondary slow phase represents B_<12> binding on newly synthesized receptors. Both phases of uptake seem to be controled by the same receptor. The half-life of the free receptor is estimated to be 66 min whereas the B_<12> receptor complex is very stable.
日本植物生理学会の論文