Tyrosyl-tRNA synthetase: a potential kyotorphin synthetase in mammals

抄録

Kyotorphin, an opioid-like analgesic dipeptide (L-tyrosyl-L-arginine) discovered from bovine brain, has neuromodulator roles in terms of synaptosomal localization, existence of specific receptor and biosynthetic enzyme activities in the brain. Regarding biosynthesis, it was found that kyotorphin is formed in the brain from its constituent amino acids, L-tyrosine and L-arginine, by enzyme termed kyotorphin synthetase, but its molecular identification has remained to be performed. The levels of human tyrosyl-tRNA synthetase (TyrRS) transcripts were highest in the pancreas and higher in testis, skeletal muscle adrenal gland and brain regions. Biochemical characterization of recombinant TyrRS revealed that kyotorphin is synthesized from tyrosine, arginine and ATP in the presence with Km 1,400 μM and 200 μM for arginine and tyrosine, respectively. The treatment of PC12 cells with siRNA for rat TyrRS in the presence of 1.6 mM tyrosine, arginine, proline or tryptophan, significantly reduced the levels of kyotorphin, but not those of tyrosine-tyrosine, tyrosine-proline or tyrosine-tryptophan in the LC-MS/MS measurement. The mouse TyrRS level was more abundant in midbrain and medulla oblongata than the other brain regions. When 1000 mg/kg (p.o.) arginine was administered 2 h prior to brain dissection, the kyotorphin levels in midbrain, medulla oblongata and olfactory bulb were significantly increased, though basal kyotorphin levels were relatively even throughout brain regions. The arginine administration also caused analgesia in a kyotorphin-receptor antagonist-reversible manner. All these findings suggest that TyrRS could be a most probable candidate for kyotorphin synthetase.