Review: Using physiologically based models to predict population responses to phytochemicals by wild vertebrate herbivores.

To understand how foraging decisions impact individual fitness of herbivores, nutritional ecologists must consider the complex in vivo dynamics of nutrient-nutrient interactions and nutrient-toxin interactions associated with foraging. Mathematical modeling has long been used to make foraging predictions (e.g. optimal foraging theory) but has largely been restricted to a single currency (e.g. energy) or using simple indices of nutrition (e.g. fecal nitrogen) without full consideration of physiologically based interactions among numerous co-ingested phytochemicals. Here, we describe a physiologically based model (PBM) that provides a mechanistic link between foraging decisions and demographic consequences. Including physiological mechanisms of absorption, digestion and metabolism of phytochemicals in PBMs allows us to estimate concentrations of ingested and interacting phytochemicals in the body. Estimated phytochemical concentrations more accurately link intake of phytochemicals to changes in individual fitness than measures of intake alone. Further, we illustrate how estimated physiological parameters can be integrated with the geometric framework of nutrition and into integral projection models and agent-based models to predict fitness and population responses of vertebrate herbivores to ingested phytochemicals. The PBMs will improve our ability to understand the foraging decisions of vertebrate herbivores and consequences of those decisions and may help identify key physiological mechanisms that underlie diet-based ecological adaptations.

Photochemical cycle and light-dark adaptation of monomeric and aggregated bacteriorhodopsin in various lipid environments.

Spectral changes of bacteriorhodopsin (BR) reflecting its photochemical cycle and light-dark adaptation were monitored in order to study the effect of protein-protein and protein-lipid interactions on these reactions. For this purpose, the light-driven proton pump BR was reconstituted with various lipids, i.e., dimyristoyl- and dipalmitoyl-phosphatidylcholine, soybean phospholipids, and diphytanoyllecithin. In these vesicle systems, BR is monomeric above the lipid phase transition and above molar lipid to BR ratios of about 80. Well below the phase transition, BR is aggregated in a hexagonal lattice as in the purple membrane. This allows, on the one hand, comparison of monomeric and aggregated BR in the respective vesicle systems and, on the other hand, comparison of reconstituted BR with BR in the native purple membrane. The photoreaction cycle of all-trans-BR accompanying proton translocation proceeds via the same intermediates in the monomeric and aggregated pigment. Furthermore, both the rate and the activation energy for the decay of the cycle intermediate M-410 are independent of the aggregation state. From the results, we conclude that the functional unit responsible for BR's photocycle is the monomer itself. This is in accordance with previous observations that BR monomers are able to translocate protons during illumination [Drencher, N. A., & Heyn, M.P. (1979) FEBS Lett. 108, 307-310]. The light-dark adaptation reaction, however, is affected by BR's aggregation state. In the case of the monomer, the extent of light adaptation, i.e., the fraction of BR molecules containing 13-cis-retinal as chromophore which is converted by illumination to the respective pigment with the all-trans isomer, is reduced by 50% or more, and the rate of dark adaptation is slowed down about 2.5 times. For these properties too, the monomer is functional, but with a reduced efficiency. This indicates regulatory control by neighboring BR molecules. The rate of the photocycle as well as of dark adaptation is strongly affected by the chemical nature of the lipids used for reconstitution but not by the physical state of the lipid phase.

Low temperature photoacoustic spectroscopy of the purple membrane of Halobacterium halobium.

The photoacoustic spectra of light- and dark-adapted bacteriorhodopsin in aqueous and mixed water-glyc-erol (1:2) suspensions are studied in the 90-300 K temperature range. Photochemical transients are observed and interpreted in terms of the known photocycles of trans- and 13-cis-bacteriorhodopsin. The spectra differ from those obtained by low temperature optical absorption spectroscopy of glassy glycerol-water suspensions.