REGULATORY SIGNIFICANCE OF TRANSFER RNA CHARGING LEVELS .I. MEASUREMENTS OF CHARGING LEVELS IN LIVERS OF CHOW-FED RATS, FASTING RATS, AND RATS FED BALANCED OR IMBALANCED MIXTURES OF AMINO ACIDS

1. 1. To evaluate the possible role of tRNA charging levels (percent of a tRNA class which is acylated at the acceptor site) in the control of liver anabolism, an assay for the level of charging was developed based on the ability of IO4-to inactivate uncharged but not charged tRNA. Charging levels of tRNASer and tRNAThr could not be assayed owing to oxidation of the amino acyl moieties and attendant deacylation, and that of tRNACys could not be assayed owing to a high and erratic blank. The remaining charging levels were systematically somewhat underestimated by the assay. 2. 2. Evidence was obtained that charging levels in fed or fasting (24 h) rats were all near 100%. The assay was not precise enough to detect any effects of brief fasting. Tube feeding of amino acid mixtures lacking certain amino acids singly caused the charging level of the tRNA corresponding to the missing amino acid to fall considerably below that seen in the fasting rat. This provides some insight into the events leading to suppressed consumption of diets in which single amino acids are severely limiting for growth. Tryptophan deficiency produced the most striking charging level effect of those observed. The apparent high sensitivity of tRNATrp charging level to dietary tryptophan may account in part for the peculiarly deleterious effects of tryptophan deficiency on hepatic polysome aggregation and activity. © 1969.