Effect of tryptophan on enzymes and proteins of hepatic nuclear envelopes of rats.

It has been demonstrated earlier that the administration of tryptophan to fasted animals increased the levels of mRNA in the cytoplasm of the liver by stimulating the translocation of nuclear poly(A)-mRNA into the cytoplasm. Also, tryptophan increased the activity of hepatic nuclear envelope (NE) nucleoside triphosphatase, an enzyme considered to be involved in nucleocytoplasmic translocation of mRNA. In this study, the activities of two other NE-associated enzymes, protein phosphokinase and phosphoprotein phosphohydrolase, also implicated in nuclear RNA transport, were investigated in the livers of rats that received a single tube feeding of tryptophan. The administration of tryptophan to fasted rats 10 minutes before killing increased the hepatic NE activities of both enzymes, protein phosphokinase and phosphoprotein phosphohydrolase. Furthermore, tryptophan administration increased the in vivo incorporation of 3H-leucine into NE proteins (+83%) and into other subcellular fractions (+34 to +43%) of the liver compared with that into corresponding fractions of the control rats. Rats that received 3H-leucine to prelabel hepatic proteins and then were treated with puromycin to inhibit further protein synthesis followed by tube feeding of tryptophan revealed greater radioactivity associated with NE proteins than that in controls. These findings suggest that tryptophan may act to stimulate the transport or availability of proteins to the vicinity of the NE, possibly specific regulatory proteins, such as nucleoside triphosphatase, protein phosphokinase and phosphoprotein phosphohydrolase, which show an increase in activity and may then be responsible for the increase in the rate of nucleocytoplasmic translocation of mRNA.