Feedback inhibition of polyisoprenyl pyrophosphate synthesis from mevalonate in vitro. Implications for protein prenylation.

The prenylation of proteins utilizes the polyisoprenyl pyrophosphates (FPP) and geranylgeranyl pyrophosphate (GGPP) as prenyl donors. These polyisoprenoids are also precursors to ubiquinone and dolichol synthesis. We have previously described the geranylgeranylation of rab 1b from labeled mevalonate in rabbit reticulocyte lysates (Khosravi-Far, R., Lutz, R. J., Cox, A. D., Conroy, L., Bourne, J. R., Sinensky, M., Balch, W. E., Buss, J. C., and Der, C. J. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 6264-6268). We now directly demonstrate the incorporation of mevalonate into FPP and GGPP in rabbit reticulocyte cytosol. High pressure liquid chromatography analysis reveals that only all-trans-E,E,E-GGPP, the prenyl donor for in vivo protein geranylgeranylation, is synthesized. Incubations with recombinant H-ras and rab1b result in an increased synthesis of farnesyl and geranylgeranyl derivatives, respectively. The increase is wholly accounted for by protein-incorporated polyisoprenoids with no change in the polyisoprenyl pyrophosphate pools. Further, GGPP inhibits its own synthesis, without affecting FPP synthesis, with half-maximal inhibition at approximately 3 microM GGPP. Inhibition of FPP synthesis by the inhibition of isopentenyl isomerase causes a dramatic increase in isopentenyl pyrophosphate synthesis. FPP also inhibits conversion of mevalonate into FPP. These findings indicate that these polyisoprenyl pyrophosphates can down-regulate their own synthesis in vitro, and this regulation may control the levels of these polyisoprenoids in vivo.     

Apparent anion imbalance in the fresh water adapted eel

Summary On adapting brackish waterAnguilla anguilla to fresh water it was noted that, while the plasma sodium, magnesium,pCO₂ and pH were held reasonably constant, there was a substantial fall in chloride concentration (–33 mEq). The gradient of the linear correlations between plasma sodium and chloride also fell (brackish water gradient=0.92, fresh water gradient=0.21) indicating that a new pattern of plasma ion interrelationships was being established. Comparison with plasma Na/Cl ion ratios from other fishes suggested that this phenomenon was peculiar toA. anguilla. Corresponding with the very low plasma chloride levels plasma bicarbonate was four to five times that found in other fishes, and this was thought related to the finding that the haematocrit value almost doubled during adaptation to fresh water. In fresh water adapted fish a fall in plasma chloride was associated with a rise in plasma bicarbonate, however the charge compensation effect of this response was only partial as summing the common plasma cations and anions left an anion deficit of about 34 mEq to be accounted for.