Microbial oxidation of protoporhydrinogen, an intermediate in heme and chlorphyll biosynthesis.

The oxidation of protoporphyrinogen to protoporphyrin, a late step in heme and chlorophyll synthesis, is catalyzed aerobically by a particulate fraction of Escherichia coli at a rate significantly higher than the rate of autooxidation. This activity is heat labile and is markedly inhibited by addition of respiratory substrates such as NADH. NADH is oxidized at a rate 100-fold higher than protoporphyrinogen. Particles from a cytochrome-less mutant of E. coli were markedly deficient in protoporphyrinogen oxidizing activity. Particles from a quinone-deficient mutant were also deficient. These findings suggest a possible role for the electron transport system in aerobic protoporphyrinogen oxidation. This activity was also examined in a variety of other bacteria. Particles from Streptococcus faecalis, which does not synthesize heme, were unable to oxidize protoporphyrinogen, confirming the specificity of this activity. Particles from aerobically grown Staphylococcus aureus exhibited protoporphyrinogen oxidizing activity, but particles from anaerobically grown cells had no activity above that of the nonenzymatic control. This indicates the repressible nature of this activity, and may also explain why Staphylococci synthesize cytochromes during aerobic, but not during anaerobic growth. Particles from photosynthetically grown Rhodopseudomonas spheroides, which contain both chlorophyll and heme, oxidized protoporphyrinogen at a rate no higher than the nonenzymic control. However, particles from cells grown aerobically, when bacteriochlorophyll synthesis is markedly repressed, readily exhibited protoporphyrinogen oxidizing activity. These initial findings suggest that this activity is detectable in cells primarily synthesizing heme, but not in cells primarily synthesizing bacteriochlorophyll, and could have implications both for the mechanism and regulation of the heme and bacteriochlorophyll pathways.     

A high-pressure sample cell for circular dichroism studies.

The construction and operation of a preparative polyacrylamide-gel electrophoresis system is described. Emerging protein bands are collected by an intermittent pumping system which is based on the design of Brownstone ((1969) Anal. Biochem.27, 25–46). The original pressure-sensitive operation was, however, simplified to time-volume operation. Cooling of the gel by a central cooling finger, essentially according to Jovin et al. ((1964) Anal. Biochem.9, 351–369), has also been added. To accomodate the polyethylene tubing needed for intermittent collection of protein and also the central cooling finger, it is necessary to polymerize the gel in a mold before it is installed in the gel housing compartment of the electrophoresis cell. Gel concentrations of 5% and higher can be used in this system. Dilution of emerging protein samples by the intermittent collection system is kept to a minimum. This fact, together with simplicity of design makes it suitable for general preparative work with polyacrylamide-gel electrophoresis. Operation of the apparatus and resolution of protein bands are demonstrated by separation of bovine serum albumin polymers and thyroxine-binding proteins in human serum.     

The mobile receptor hypothesis and "cooperativity" of hormone binding. Application to insulin.

The mobile receptor hypothesis has been proposed to describe the process by which hormone receptor binding initiates a biological response; it states that receptors, which can diffuse independently in the plane of the membrane, reversibly associate with effectors to regulate their activity. The affinity for effector is greater when the receptor is occupied by hormone. A mathematical expression of the mobile receptor hypothesis is used to show that: (1) The predicted kinetics of hormone receptor binding may be indistinguishable from "negative cooperativity." (2) Receptor occupancy and biological response may be coupled in a non-linear fashion. By choosing specific parameters, most of the existing data on insulin binding and biological responses can be explained in terms of the mobile receptor hypothesis. Thus, the following are easily explained: (1) A single homogeneous receptor may appear kinetically to be composed of two classes (of high and low affinity) of receptors. (2) Occupancy of the apparent class of high affinity receptors is related linearly to the biological response. (3) The same receptor in different tissues may appear to have different affinity. (4) The binding of different biologically active insulin analogues may exhibit different degrees of "cooperativity." These considerations may also be pertinent to interpretations of other hormone-receptor systems and of various ligand-macromolecule interactions.     

Electrophysiological evidence for rod and cone-based vision in the nocturnal flying squirrel

Summary The flying squirrel (Glaucomys volans) is a strongly nocturnal rodent. Previous anatomical observations suggested that the retina of this animal contains some cone-like receptors in addition to large numbers of rods. Evidence for duplicity of function in this visual system was obtained from an examination of three indices of visual activity: the electroretinogram (ERG), the isolated PIII retinal response, and the visually evoked cortical potential (VECP). The spectral sensitivity of the dark-adapted flying squirrel is similar to that of other mammals — it has a 500 nm peak (Figs. 3, 8). Responses of the ERG and isolated PIII to flickering light indicate the operation of two processes (Figs. 4, 7), one of which is unable to follow flickering light at repetition rates above 10–15 Hz. Spectral sensitivity measurements reveal that these two processes have different spectral sensitivities. The photopic mechanism in the flying squirrel visual system has peak sensitivity at about 520 nm (Figs. 5, 7, 9). The effects of steady light adaptation are much more obvious in the cortical potentials than they are in the retinal potentials.We thank David Birch for his advice and assistance. This research was supported by a Grant from the National Eye Institute (EY-00105).     

A biostratigraphic zonation of the middle and upper siwaliks of the potwar plateau of Northern Pakistan

Two biostratigraphic reference sections are designated for the middle and upper Siwalik formations of northern Pakistan. The stratigraphic ranges of 41 mammalian taxa are established in the reference sections and the two sections are correlated to each other and to the standard Magnetic Polarity Time Scale by the magnetic-polarity stratigraphy. The resulting stratigraphic and magnetostratigraphic framework is used to define four contiguous biostratigraphic interval-zones. The proposed interval-zones are bounded by five important faunal events which are identified in the reference sections. The four proposed interval-zones and the approximate ages in the reference sections of their lower and upper boundaries are: (1) the “Hipparion s.l.” Interval-Zone 7.4–9.5 m.y. B.P.; (2) the Selenoportax lydekkeri Interval-Zone 5.3–7.4 m.y. B.P.; (3) the Hexaprotodon sivalensis Interval-Zone 5.3-2.9 m.y. B.P.; and (4) the Elephas planifrons Interval-Zone 2.9-1.5 m.y. B.P.The paleomagnetic and biostratigraphic evidence indicates that the Tatrot Beds, from which Pilgrim developed his “Tatrot Faunal Zone”, are between 2.5 and 3.4 m.y. B.P., with a preferred maximum age of 2.9 m.y. B.P. The biostratigraphic evidence indicates the Sethi Nagri locality (Y311)—the primary source of Pilgrim's “Nagri Faunal Zone”— is in the “Hipparion s.l.” Interval-Zone. The paleomagnetic evidence suggests an approximate age of 9 m.y. B.P. A major faunal event at 9.5 m.y. B.P. introduced equids, suids, and large giraffes from Eurasia into the Indain subcontinent. A second, more diffuse faunal event between 5.3 and 2.9 m.y. B.P. introduced several African taxa as well as Equus and cervids from Eurasia.     

Historical biogeography of tamarins,genus Saguinus: the molecular phylogenetic evidence

Hypotheses of the historical biogeography of tamarins (genus Saguinus) based on variation in coat colors and body size are tested using phylogenetic relationships inferred from mitochondrial DNA (mtDNA) sequence data. Samples from all 12 species of Saguinus and several subspecies are included in the analysis. Approximately 1,200 bases of mtDNA sequence from the cytochrome b and D-loop regions are reported for the tamarins and several outgroup taxa. Parsimony analysis of the mtDNA sequence data reveals Saguinus to be a monophyletic taxon composed of two major clades: one, the Small-bodied clade, contains S. nigricollis, S. tripartitus, and S. fuscicollis, and the other, the Large-bodied clade, contains the other nine species. The phylogenetic relationships among tamarins inferred from the mtDNA sequence data reject previous hypotheses for the historical biogeography of tamarins and suggest different dispersal routes for this group of New World monkeys. The molecular data suggest that tamarins dispersed across South America in two major waves from an origin somewhere south of the Amazon. One wave moved in a westerly direction, whereas the other moved in a northeastern direction toward the Amazon delta and then west along the northern portion of the continent into northern Colombia and Panama. Am J Phys Anthropol 108:65–89, 1999. © 1999 Wiley-Liss, Inc.     

Population genomics and geographic dispersal in Chagas disease vectors: Landscape drivers and evidence of possible adaptation to the domestic setting

Accurate prediction of vectors dispersal, as well as identification of adaptations that allow blood-feeding vectors to thrive in built environments, are a basis for effective disease control. Here we adopted a landscape genomics approach to assay gene flow, possible local adaptation, and drivers of population structure in Rhodnius ecuadoriensis, an important vector of Chagas disease. We used a reduced-representation sequencing technique (2b-RADseq) to obtain 2,552 SNP markers across 272 R. ecuadoriensis samples from 25 collection sites in southern Ecuador. Evidence of high and directional gene flow between seven wild and domestic population pairs across our study site indicates insecticide-based control will be hindered by repeated re-infestation of houses from the forest. Preliminary genome scans across multiple population pairs revealed shared outlier loci potentially consistent with local adaptation to the domestic setting, which we mapped to genes involved with embryogenesis and saliva production. Landscape genomic models showed elevation is a key barrier to R. ecuadoriensis dispersal. Together our results shed early light on the genomic adaptation in triatomine vectors and facilitate vector control by predicting that spatially-targeted, proactive interventions would be more efficacious than current, reactive approaches.     

Modelling brain representations of abstract concepts

Abstract conceptual representations are critical for human cognition. Despite their importance, key properties of these representations remain poorly understood. Here, we used computational models of distributional semantics to predict multivariate fMRI activity patterns during the activation and contextualization of abstract concepts. We devised a task in which participants had to embed abstract nouns into a story that they developed around a given background context. We found that representations in inferior parietal cortex were predicted by concept similarities emerging in models of distributional semantics. By constructing different model families, we reveal the models’ learning trajectories and delineate how abstract and concrete training materials contribute to the formation of brain-like representations. These results inform theories about the format and emergence of abstract conceptual representations in the human brain.     

Spectral sensitivity and photopigments of a nocturnal prosimian,the bushbaby (Otolemur crassicaudatus)

Earlier studies yielded conflicting conclusions on the types of photoreceptors and photopigments found in the eyes of nocturnal prosimians. In this investigation a noninvasive electrophysiological procedure, electroretinogram flicker photometry, was employed to measure scotopic and photopic spectral sensitivity in the thick-tailed bushbaby (Otolemur crassicaudatus). The scotopic spectral sensitivity function of the bushbaby has a peak of about 507 nm. Under photopic test conditions, spectral sensitivity shifts toward the longer wavelengths. The results from a series of adaptation experiments indicate that the cones of the bushbaby retina contain only a single type of cone photopigment (peak sensitivity at about 545 nm). One implication from this result is that these animals do not have color vision. The photopigment arrangement of the bushbaby is different from that earlier found in diurnal and crepuscular prosimians but is similar to that of the owl monkey, the only nocturnal simian. © 1996 Wiley-Liss, Inc.     

Glutathione-dependent oxidative modification of protoporphyrin and other dicarboxylic porphyrins by mammalian and plant peroxidases.

Protoporphyrin, an intermediate in heme and chlorophyll biosynthesis, can accumulate in human and plant tissues under certain pathological conditions and is a photosensitizer used in cancer phototherapy. We previously showed that protoporphyrin and the related non-natural dicarboxylic porphyrin deuteroporphyrin are rapidly oxidized by horseradish peroxidase in the presence of some thiols, especially glutathione. This study reports that bovine lactoperoxidase, but not leucocyte myeloperoxidase, can also catalyze this reaction and that Tween and ascorbic acid are inhibitors. Exogenous hydrogen peroxide is not required and cannot replace glutathione. Deuteroporphyrin was oxidized to a unique green chlorin product with two oxygen functions added directly to the characteristic reduced pyrrole ring of the chlorin. Spectroscopic and chromatographic results suggest that protoporphyrin was oxidized not to a green chlorin, but to a much more polar red porphyrin modified by oxidative addition to the two vinyl side chains. Two related nonnatural dicarboxylic porphyrins, with ethyl or hydroxyethyl instead of vinyl side chains, are not substrates or products for this enzymatic conversion.