Abstract
Pulse labeling of adipose tissue and liver by injection of [Me 3H]thymidine in developing rats receiving 20% dietary lipids suggests a dual function of this molecule in nuclei: tracer incorporation into newly formed DNA, and tracer incorporation into RNA linked to nascent DNA. This conclusion is based on (1) determination of the specific radioactivity (SRA) of long alkali-stable nucleotide sequences which increased dramatically during 1 hr after radioactive injection when rats were given 20% lard (L) containing little linoleic acid, and decreased sharply thereafter; (2) isopycnic centrifugation of the native DNA extracted from nuclei pulse labeled in vivo with [Me 3H]thymidine and [6- 14C]orotic acid, which indicated the presence of doubly-labeled constituents with the buoyant density of DNA; (3) isopycnic centrifugation after freeze-thawing of the same samples, which indicated the disappearence of 14C-labeled long alkali-stable nucleotide sequences in the tritium labeled material with the buoyant density of DNA. A transient elevated level of radioactivity was also observed by autoradiography and determination of labeling index in adipose cell nuclei. The labeling index was 2.3-fold higher 1 hr after injection of [Me- 3H]thymidine in (L) group adipose tissues than in (SO) adipose tissues, when rats were given 20% sunflower oil containing large proportion of linoleic acid. In contrast, the labeling index in (SO) nuclei was 1.8-fold higher than in (L) nuclei 2 hr after injection. The labeling index was much lower in nuclei of both groups 3 hr after injection. Moreover, the striking increase in SRA was not observed in nuclei after injection [6- 3H]thymidine instead of [Me- 3H]thymidine, and was never observed in mitochondria, whichever labeled thymidine was injected. Therefore, high lipid intake appears to increase methyl group requirement, and this effect seems to be more pronounced when essential fatty acid is given in small proportion. This relative methyl deficit seemed to be filled up in RNA linked to nascent DNA, by incorporation of labeled thymine during the early process of replication in adipose cells and liver of developing rats.
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