Effects of lithium on tryptophan transport into plasma-membrane vesicles derived from rat brain synaptosomes

Abstract

during the period 5-120min after injection of thiamin. Most of the thiamin in the brain was in the phosphorylated form: free (unphosphorylated) thiamin 3%. thiamin monoand pyrophosphates 90%. and thiamin triphosphate 2%. The ratio of distribution of thiamin pyrophosphate to thiamin monophosphate was I : I . but values of 3.5 : 1 have also been obtained. The factors responsible for these variations require investigation. The radioactivity in the rain at 5min after injection of 1I4Clthiamin was found only in thiamin and thiamin pyrophosphate. giving evidence of phosphorylation of thiamin directly t o thiamin pyrophosphate. It increased linearly with time from 10 to 30min and thereafter showed a steady decline in the rate of uptake of I i4CIthiamin. Free thiamin accounted for 20% of the total radioactivity in the brain at Smin, 30% at IOmin. 24% at 20min. 20% at 30min, 1 I % at 60min and 8% at 120min. Thiamin monoand pyro-phosphates together contained 72-82% and thiamin triphosphate 4% of the radioactivity in the brain at times from 10 to 120min after injection of 1 14C Ithiamin. Thiamin showed the highest specific radioactivity (d.p.m./ nmol) at all times: its phosphate esters appeared to approach this value at 120min (Fig. I). In view of the fact that thiamin pyrophosphate is the precursor of the other two thiamin phosphates, the observed specific radioactivities were contrary to expectation: thiamin triphosphate > thiamin monophosphate > thiamin pyrophosphate. Moreover, the specific-radioactivity-time curves did not intersect during the period 5-120min after injection of 1Clthiamin. These findings are at variance with the precursor-product relationship and indicate the presence of more than one compartment of the thiamin pyrophosphate pool.