1-Carboxyallenyl phosphate, an allenic analogue of phosphoenolpyruvate

1-Carboxyallenyl phosphate, the allenic homologue of phosphoenolpyruvate, has been synthesized in six steps. The key step in the synthesis is the isomerization of methyl 2-hydroxy-3-butynoate to the corresponding allenol and phosphorylation of this material. The allene is an excellent substrate for pyruvate kinase, undergoing reaction at more than half the rate of phosphoenolpyruvate. The allene is also a substrate for phosphoenolpyruvate carboxylase, being hydrolyzed by the enzyme rather than carboxylated. With both enzymes, the organic product is 2-oxo-3-butenoate, which gradually inactivates the enzymes by reaction with one or more sulfhydryl groups not at the active site.     

Biochemical pathways in prokaryotes can be traced backward through evolutionary time

For the first time, a credible prokaryotic phylogenetic tree is being assembled by Woese and others using quantitative sequence analysis of oligonucleotides in the highly conservative rRNA. This provides an evolutionary scale against which the evolutionary steps that led to the arrangement and regulation of contemporary biochemical pathways can be measured. This paper presents an emerging evolutionary picture of aromatic amino acid biosynthesis within a large superfamily assemblage of prokaryotes that is sufficiently developed to illustrate a new perspective that will be applicable to many other biochemical pathways.      

Posttetanic potentiation of human dorsiflexors

O'Leary, Deborah D., Karen Hope, and Digby G. Sale.Posttetanic potentiation of human dorsiflexors.J. Appl. Physiol. 83(6):2131-2138, 1997.Twitch contractions of the ankle dorsiflexors were evoked before and after applied 7-s tetanic stimulation at 100 Hzin 20 young adults. Torque decreased 15% during the tetanus. At 5 safter tetanus, twitch peak torque had potentiated 45%. Potentiationdeclined to 28% after 1 min, rose slightly to 33% at 2 min, anddeclined slowly with potentiation still 25% after 5 min. There waslarge intersubject variation in the amount of potentiation(5-140%) and its persistence (5 to 20 min). The muscle compoundaction potential (M wave) did not change significantly (from pretetanicvalue) at 5 s after tetanus but increased sharply (26%) at 2 min andthen subsided. Twitch half relaxation time (23%) decreasedsignificantly more than twitch rise time (13%) 5 s after tetanus andrecovered more slowly. Twitch rates of torque development (75%) andrelaxation (71%) increased similarly 5 s after tetanus and were stillelevated (~25%) at 5 min. The extent of twitch torque potentiationwas significantly inversely correlated with pretetanic twitch rise time(r = 0.69), half relaxation time (r = 0.61), andtwitch-to-tetanus ratio (r = 0.66). The data indicate that posttetanic potentiation has agreater effect on twitch half relaxation time than on time to peaktorque and is more prominent in muscles with a short twitch time courseand small twitch-to-tetanus ratio.

     

Simulation of vestibular semicircular canal responses during righting movements of a freely falling cat

The righting maneuver of a freely falling cat was filmed at 1000 pictures per second, and the head position about the roll axis was digitized from each film frame using a graphics input tablet. The head angular velocity and acceleration were computed from the roll axis position trajectory. Head acceleration trajectories approximated two periods of a damped sinusoid at a frequency of 26 Hz. Head acceleration peak amplitudes exceeded 120,000 deg/s². These trajectories were used as stimuli for the horizontal semicircular canals in a computer simulation of first-order afferent responses during the fall. Linear system afferent response dynamics, characterized in a previous study of the cat horizontal canal using pseudorandom rotations, provided the basis for linear predictions of falling cat afferent responses. Results showed predicted single afferent firing rates that exceeded physiological values; and variations in afferent sensitivities and phase were predicted among different neurons. Fast head movement information could be carried by ensemble populations of vestibular neurons, and a phase-locking encoding hypothesis is proposed which accomplishes this. Implications for central program versus peripheral vestibular feedback strategies for motor control during falling are presented and discussed.     

Decarboxylation-dependent transamination catalyzed by mammalian 3,4-dihydroxyphenylalanine decarboxylase

In addition to the usual decarboxylation, pig kidney 3,4-dihydroxyphenylalanine (dopa) decarboxylase catalyzes a decarboxylation-dependent transamination which converts dopa into 3,4-dihydroxyphenylacetaldehyde and sinultaneously converts enzyme-bound pyridoxal-P into pyridoxamine-P. Similar reactions occur when this enzyme acts on m-tyrosine, alpha-methyldopa, and alpha-methyl-m-tyrosine. The transamination occurs in about 0.02% of decarboxylations of dopa and m-tyrosine and in about 2% of decarboxylations of alpha-methyldopa and alpha-methyl-m-tyrosine. The fraction of decarboxylations proceeding by the transamination pathway is independent of pH. This reaction appears to result from a divergence in the normal mechanism of decarboxylation; the quinoid intermediate which is formed by decarboxylation of the substrate-pyridoxal-P-Schiff base ordinarily protonates on the alpha carbon of the amino acid, but protonation occasionally occurs at the benzylic carbon of the coenzyme, and this latter route leads to transamination.     

Alcohol production by selected yeast strains in lactase-hydrolyzed acid whey

Ethanol production by Kluyveromyces fragilis and Saccharomyces cerevisiae was studied using cottage cheese whey in which 80 to 90% of the lactose present had been prehydrolyzed to glucose and galactose. Complete fermentation of the sugar by K. fragilis required 120 hr at 30°C in lactase-hydrolyzed whey compared to 72 hr in nonhydrolyzed whey. This effect was due to a diauxic fermentation pattern in lactase-hydrolyzed whey with glucose being fermented before galactose. Ethanol yields of about 2% were obtained in both types of whey when K. fragilis was the organism used for fermentation. Saccharomyces cerevisiae produced alcohol from glucose more rapidly than K. fragilis, but galactose was fermented only when S. cerevisiae was pregrown on galactose. Slightly lower alcohol yields were obtained with S. cerevisiae, owing to the presence of some lactose in the whey which was not fermented by this organism. Although prehydrolysis of lactose in whey and whey fractions is advantageous in that microbial species unable to ferment lactose may be utilized, diauxie and galactose utilization problems must be considered.