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The balance of photogeneration and recombination gives rise to an optimum bandgap for any solar cell. The radiative limit represents the lowest permissible level of recombination in a solar cell and, therefore, places an upper limit on the voltage that can be attained. Introducing additional nonradiative recombination results in a loss in voltage that can only be compensated for by moving to higher bandgaps. Consequently, the optimal bandgap for solar energy conversion will rise with increasing nonradiative recombination rate. This balance was recognized by Shockley and Queisser for single-junction solar cells and is here extended to multijunction solar cells. A rise in optimal bandgaps has been observed in simulated single-, double-, and triple-junction devices as nonradiative recombination increases. Optimal bandgaps between excellent and poor diode quality devices are shown to differ by 100s of meV under 1-sun illumination with both terrestrial and extraterrestrial spectra but exhibit no significant change at high concentration due to the dominance of the radiative component in the recombination dynamics.

The unparalleled peroxisome-to-mitochondrion mistargeting of alanine:glyoxylate aminotransferase (AGT) in the hereditary disease primary hyperoxaluria type 1 is caused by the combined presence of a common Pro11 --> Leu polymorphism and a disease-specific Gly170 --> Arg mutation. The Pro11 --> Leu replacement generates a functionally weak N-terminal mitochondrial targeting sequence (MTS), the efficiency of which is increased by the additional presence of the Gly170 --> Arg replacement. AGT dimerization is inhibited in the combined presence of both replacements but not when each is present separately. In this paper we have attempted to identify the structural determinants of AGT dimerization and mitochondrial mistargeting. Unlike most MTSs, the polymorphic MTS of AGT has little tendency to adopt an alpha-helical conformation in vitro. Nevertheless, it is able to target efficiently a monomeric green fluorescent (GFP) fusion protein, but not dimeric AGT, to mitochondria in transfected COS-1 cells. Increasing the propensity of this MTS to fold into an alpha-helix, by making a double Pro11 --> Leu + Pro10 --> Leu replacement, enabled it to target both GFP and AGT efficiently to mitochondria. The double Pro11 --> Leu + Pro10 --> Leu replacement retarded AGT dimerization in vitro as did the disease-causing double Pro11 --> Leu + Gly170 --> Arg replacement. These data suggest that N-terminal alpha-helix formation is more important for maintaining AGT in a conformation (i. e. monomeric) compatible with mitochondrial import than it is for the provision of mitochondrial targeting information. The parallel effects of the Pro10 --> Leu and Gly170 --> Arg replacements on the dimerization and intracellular targeting of polymorphic AGT (containing the Pro11 --> Leu replacement) raise the possibility that they might achieve their effects by the same mechanism.

The metabolism of the methyl group of 5-methyltetrahydrofolate was studied in rats in which cobalamin had been inactivated by exposure to nitrous oxide and in air-breathing control animals. Methylfolate labeled with [14C] in the methyl group and with [3H] in the pteridine- PABA portion was injected and the disappearance of [14C]H3- relative to [3H]folate was measured in liver. The half-time of the methyl group in the livers of control rats was two hours. There was no turnover of the methyl group for the first 72 hours after cobalamin inactivation. After 72 hours, there was a slow turnover of the methyl group, with a half- time of 43 hours. In control rats, it is assumed that the methyl group was metabolized by transfer to homocysteine to form methionine. In cobalamin-inactivated rats, it was shown that methylfolate was used as the substrate for forming folate polyglutamate, and analogues with 3, 4, and 5 glutamic acid residues were present. It is likely that oxidation of the methyl group by methylene tetrahydrofolate reductase occurs from folate polyglutamate containing six and seven glutamic acid residues, (Brody et al, Biochemistry 21: 276, 1982), since we were unable to demonstrate labeled methyl in longer chain analogues.

Rats exposed to N20 show a decrease in liver folate to about 25% of the initial value after 10 days. There is a transient increase in the amount of 5-methylterrahydropteroylpolyglutamate in the first 24 h, but thereafter the content decreases. The level of 5-methyltetrahydropterolymonoglutamate declines without any transitory increase. The transient accumulation of 5-methyltetrahydropteroylpolyglutamate is due to failure of methionine synthetase. Thereafter the decrease in the amount of methylfolate makes it improbable that trapping of methylfolate is the explanation for failure of folate metabolism in vitamin B12 deficiency.

Cultured intact fetal membrane disks initially produced high levels of prostaglandin E2 (PGE2) on the fetal and maternal sides which declined during four days of culture. The transfer of a bolus of PGE2 from the fetal side to the maternal side of the membrane ranged from 1% to 3% after 24 hours of culture, and was a minimum over the period of 48-72 hours from the start of the incubation. To assess the handling of PGE2 synthesised by the amnion, 3H-arachidonic acid was incorporated into cultured amnion and into the amnion side of cultured intact fetal membrane disks. Labelled amnion released 3H-PGE2 on both sides of the tissue, whereas similarly labelled cultured intact fetal membrane only had detectable levels of 3H-PGE2 on the fetal side. It was calculated that no more than 9.7 ± 1.4% of the PGE2 synthesised by the amnion crossed to the maternal side of the membrane without being metabolised during the transfer through the membrane. These results are consistent with similar indirect methods which suggested that PGE2 from the amnion may have only a limited role in human labor, and indicates the importance of using appropriate culture systems to investigate intra-uterine prostaglandin production.

Rats were exposed to nitrous oxide, which inactivates cob(I)alamin (Cbl). As in air-breathing rats methionine administration led to the conversion of hepatic 5-methyltetrahydrofolate (MeH4 folate) into formyltetrahydrofolate. The recovery of MeH4 folate levels in liver after its oxidation initiated by methionine was noted and the rate compared with that for air-breathing rats. Oxidation of MeH4 folate was less complete and occurred more slowly in Cbl-inactivated rats as compared with controls. However, recovery of MeH4 folate levels was more rapid in Cbl inactivation. S-Adenosylmethionine did not produce a significant change in MeH4 folate levels in Cbl-inactivated rats, whereas it did so in air-breathing animals.

N2O oxidizes and inactivates cob[I]alamin, and animals exposed in this way serve as models for cobalamin ‘deficiency’. Such animals show a fall in activity of glycinamide ribotide transformylase and a rise in that of 5-amino-4-imidazolecarboxamide ribotide transformylase. The fall in glycinamide ribotide transformylase activity was prevented by parenteral 5'-methylthioadenosine derived from methionine. Methylthioadenosine in turn is converted into formate. Activity of glycinamide ribotide transformylase recovers after 7 days despite continued N2O inhalation, and this is probably related to restoration of methionine synthesis by induction of betaine:homocysteine transmethylase.

Exposure to N2O inactivates cob[I]alamin and interferes with the activity of methionine synthetase, of which cob[I]alamin is a coenzyme. Less directly, it stops the formation of folate polyglutamate from tetrahydrofolates. Studies on the activity of folate polyglutamate synthetase in rat liver in vivo were carried out. The synthetase activity increased after exposure to N2O for up to 48 h, but longer exposure was accompanied by a return of activity to baseline values. The rise in synthetase activity was prevented by supplying methionine, 5'-methylthioadenosine or 5-formyltetrahydrofolate. The fall in folate polyglutamate synthetase activity after 48 h was accompanied by a restoration of hepatic synthesis of folate polyglutamate despite continuation of N2O exposure.

The article presents a study that examines the dynamic processes following the absorption of photons by nanorods with the use of ultrafast pump-probe spectroscopy. The experiments excite a solution of nanorods by illuminating them with a pulsed pump laser beam. Excess photon energy has been allowed in the process of multiple exciton generation (MEG) in order to be efficiently used.

Loss of cobalamin function produces profound changes in the metabolism of formate. There is impaired synthesis of formyltetrahydropteroylglutamate synthetase (CHO-H 4,PteGlu), accumulation of endogenous formate and impaired utilization of [ 14C]formate. There are contradictory reports on the effect of cobalamin inactivation on CHO-H 4PteGlu synthetase. This study confirms a significant increase in synthetase activity following cobalamin inactivation.

Exposure of rats to nitrous oxide rapidly inactivated the cytosol enzyme, methionine synthetase, but the mitochondrial enzyme, methylmalonyl CoA mutase, seemed to be unaffected, although both enzymes require vitamin B12.