Chlorophyll a biosynthetic routes and chlorophyll a chemical heterogeneity in plants

A six-branched chlorophyll a biosynthetic pathway instead of a four-branched pathway has been proposed as being responsible for the formation of chlorophyll a in green plants. The several biosynthetic routes that make up the pathway have been described as leading to the formation of ten chemically different groups of chlorophyll a species. The latter differ from one another by one or more of the following modifications: (a) by having a vinyl or ethyl group at position 4 of the macrocycle, (b) by the nature of the long-chain fatty alcohols at position 7 of the macrocycle, and (c) by having a 6-membered lactone ring instead of a 5-membered cyclopentanone ring. The chemical structure of several of the metabolic intermediates of that pathway and of some of the chlorophyll a species have now been determined by primary chemical derivatization methods coupled to spectrofluorometric, nuclear magnetic resonance and mass spectral analyses. The formation of highly organized photosynthetic membranes in which some of the chlorophyll alpha molecules are specifically oriented is ascribed to the multiplicity of chlorophyll biosynthetic routes which result in the formation of multiple chlorophyll alpha chemical species. Proper orientation of chlorophyll in the photosynthetic membranes is visualized as being controlled by peripheral group modifications that either modulate the polarity of the Chl chromophore or control the magnitude of the net positive charge on the central Mg atom. Finally it is proposed that in addition to the proper orientation of chlorophyll a, chemical heterogeneity of the chlorophyll chromophores in the photosynthetic reaction centers is mandatory for efficient charge separation, and proper vectorial electron transfer.

A sign-reversing pathway from rods to double and single cones in the retina of the tiger salamander

Signal transmission between rods and cones was studied by passing current into a rod and recording the voltage response in a nearby double or single cone and vice versa. Two types of rod-cone interaction were found. Between immediately adjacent rods and cones, passage of current into either receptor elicited in the other receptor a sustained voltage response of the same sign as the injected current. These signals were still seen in the presence of Co2+, and are probably mediated by the electrical synapses which have been seen anatomically between adjacent rods and cones. In addition to this short-range sign-preserving interaction, passing current into a rod elicited a transient sign-inverted signal in cones up to at least 80 micron from the injected rod. No such response was seen in rods for current injection into cones. This signal was greatly reduced by Co2+ ions. Hyperpolarization of the cone to about -65 mV, with about 0.1 nA current, reversed this signal, which is presumed to be mediated by a chemical synaptic input to cones. Light flashes suppressed the sign-inverted signal for a period which was longer for brighter flashes. The time of reappearance of the signal was correlated with the return of the rod and horizontal cell potentials to their dark levels. This suppression could also be produced by an annulus of light which produced no light response in the receptors at the centre of the annulus, but which did polarize horizontal cells under the centre of the annulus. The wave form of the sign-inverted signal was similar to that produced in horizontal cells by current injection into rods, but of opposite sign. If an electrode was left in a cone for some time, the normal hyperpolarizing light response diminished, leaving a depolarizing response produced, presumably, by feed-back from horizontal cells. This signal was reversed when the cone was hyperpolarized with about 0.1 nA current. These data suggest that the sign-inverted response is mediated by feed-back from horizontal cells and, assuming that depolarization increases the rate of release of horizontal cell synaptic transmitter, then the feed-back transmitter opens channels in the cone membrane whose currents have a reversal potential around -65 mV.

Effects of GABA and glycine on the distal cells of the cyprinid retina

Two putative retinal neurotransmitters, gamma-aminobutyric acid (GABA) and glycine, were applied to the isolated carp and goldfish retinas while intracellular recordings from horizontal cells and cones were made. At relatively low concentrations (0.1-1 mM), GABA consistently hyperpolarized dark-adapted H1 or L-type cone horizontal cells and cone photoreceptors, reduced light evoked responses and suppressed the on-transients. The effects of GABA on H1 horizontal cells were abolished when Co2+ was applied to the retina, indicating that GABA exerts its effects on the horizontal cells via the receptors. Thus GABA is likely to be involved in the feedback synapse between H1 horizontal cells and the cones. Low concentrations of glycine (0.1-1 mM), hyperpolarized a number of H2 or C-type horizontal cells and selectively abolished their depolarizing responses to red light; thus glycine may be involved in synaptic pathways which mediate or modulate the depolarizing responses to red flashes in C-type horizontal cells.

L-aspartate: evidence for a role in cone photoreceptor synaptic transmission in the carp retina

A number of putative neurotransmitter substances and their antagonists were applied to the carp retina while intracellular recordings from L-type cone horizontal cells were made. Of all the substances tested, L-aspartate was found to be the most potent agent in depolarizing these horizontal cells in dark-adapted, partially light-adapted, and Co2+-treated retinas. Furthermore, DL-alpha-aminoadipate, an L-aspartate antagonist, blocked the effects of both the endogenous photoreceptor transmitter and exogenously applied L-aspartate on the horizontal cells. The results suggest that L-aspartate and the natural transmitter interact with the same population of postsynaptic receptors in the horizontal cell membrane.

Budd-chiari syndrome after taking oral contraceptives. A case report and review of 14 reported cases

A case of Budd-Chiari syndrome in a young woman taking oral contraceptives is described. Her main complaints were abdominal pain and ascites without hepatosplenomegaly and the subsequent development of shock. Diagnosis was established by selective hepatic arteriogram and vena cavagram. She was treated with supportive measures, anticoagulants and neomycin. At the time of this report, the patient is slowly convalescing, taking coumadin, diuretics and Aldactone, as well as supplementary vitamins. Reviewed are 14 cases of Budd-Chiari syndrome occurring while patients were taking oral contraceptives.