Glutamate oxaloacetate transaminase isozymes of the Triticinae: dissociation and recombination of subunits

Summary A simple procedure has been developed for the dissociation of active molecules of glutamate oxaloacetate transaminase (GOT: E.C. 2.6.1.1) into protomers and for the reassociation of the subunits into active enzymes. Results of experiments in which the protomers of genetically controlled electrophoretic variants of GOT of Triticum aestivum and of several related species were dissociated and recombined in crude tissue extracts and in partially purified preparations support the hypothesis that the enzyme exists functionally as a dimer in the Triticinae.This paper is Technical Article No. 13157 of the Texas Agricultural Experiment Station.     

Specific inhibition of phototropism in corn seedlings

Geotropism was used as a control for the specificity of potential inhibitors of phototropism by the coleoptiles of corn (Zea mays) seedlings. The compounds tested fall into three categories showing: (a) no inhibition of either phototropism or geotropism (KCl); (b) nonspecific inhibition of both phototropism and geotropism (KCN); and (c) specific inhibition of phototropism (KI, NaN₃, and phenylacetic acid). Simultaneous irradiation of coleoptiles with phototropically inert light in addition to the phototropically active blue light also results in an inhibition of phototropism. Since azide, iodide, and phenylacetic acid are known to interact with flavins while a simultaneous irradiation with a phototropically inert light may depopulate the first triplet state of flavins, these data support the hypothesis that the photoreceptor pigment for phototropism in corn is a flavin.     

Familial fragility on chromosome 16 (Fra 16q22) enhanced by both interferon and distamycin A

A family with a "fragile site" at 16q22, inducible by both interferon and Distamycin A, is reported. Immunological problems were found in the family. In a sibship of ten, eight children had died in infancy. Our study led to the conclusions that interferon and Distamycin A induce fragility at the same site, which has the same characteristics as the spontaneous fragile site; that a viral hypothesis for this fragility may be supported; and that immunoincompetence of one kind or another must be considered in families presenting a fragile site at 16q22.     

Diphtheria toxin-receptor interaction: A polyphosphate-insensitive diphtheria toxin-binding domain

Inositol hexaphosphate, and other polyphosphates, inhibit diphtheria toxin-mediated cytotoxicity by binding to the toxin at a highly cationic site called the P site and preventing toxin binding to cell surface receptors. The binding of diphtheria toxin to a solubilized cell surface glycoprotein (150,000 daltons) is also inhibited by these polyphosphates. Treatment of this 150,000 dalton diphtheria toxin-binding cell surface glycoprotein with papain yielded an 88,000 dalton and a 74,000 dalton diphtheria toxin-binding glycoprotein whose binding to toxin was no longer inhibited by inositol hexaphosphate. This result suggests a model of diphtheria toxin-receptor interaction in which the toxin receptor possesses one binding site which interacts with the P site of the toxin in a $\text{polyphosphate-sensitive}$ fashion, and another binding site (located within the papain-derived 74,000–88,000 dalton glycoproteins) which can interact with the toxin at a site distinct from the P site (the X site) in a $\text{polyphosphate-insensitive}$ fashion. This X site-receptor interaction may be involved in the binding of CRM proteins that bind to the toxin receptor but that do not bind polyphosphates, or it may be involved in the entry process of the toxin.     

The influence of prey size and abundance, and individual phenotype on prey choice by the three-spined stickleback, Gasterosteus aculeatm L.

Prey selection behaviour of three-spined sticklebacks, Gasterosteus aculeatus L., was studied in two experiments. Where possible, the experimental apparatus satisfied the assumptions of the simplest optimal diet model (the basic prey model); prey were presented sequentially, the fish could not search for and handle prey at the same time, and net energy gain, handling time and encounter rate were fixed. Experiment 1 presented fish with a range of Asellus sizes so that pursuit ( p ) and handling ( h ) time could be related to prey size. Published energy values of Asellus together with pursuit and handling times were used to calculate E /( p+h ) for Asellus measuring 3,4,5,6,7 and 9 mm. Pursuit times did not differ with prey size but handling times did. E /( p+h ) was very variable particularly at the larger prey sizes. Experiment 2 presented fish with two sequences of prey differing in the encounter rate with the most profitable prey sizes. Fish did not select the diet predicted by the basic prey model tending to always ignore the largest prey even when net energy gain would have been maximized by including them in the diet. Further analysis showed that the probability of a prey size being taken was a function of prey size, fish stomach fullness and encounter rate. It is concluded that the basic prey model is too simple to capture the behaviour of the fish. One of its main faults is that the changing state of the fish through the feeding bout is ignored.     

A review of the salt sensitivity of the Australian freshwater biota

In Victoria, Australia, both dryland salinity and salinity in irrigation regions are serious agricultural problems. One option to control the latter is to pump groundwater to maintain it below the surface. However, this leaves a saline wastewater for disposal, probably into local streams or wetlands. This review of the salt sensitivity of the biota of Australian streams and wetlands gives information of interest to those responsible for developing controls on these discharges. The review addresses the lethal and sub-lethal effects of salinity on microbes (mainly bacteria), macrophytes and micro-algae, riparian vegetation, invertebrates, fish, amphibians, reptiles, mammals, and birds. Data suggest that direct adverse biological effects are likely to occur in Australian river, stream and wetland ecosystems if salinity is increased to around 1 000 mg L⁻¹. The review highlights a general lack of data on the sensitivity of freshwater plants and animals to salinity increases.     

Glycosylation of nuclear and cytoplasmic proteins. Purification and characterization of a uridine diphospho-N-acetylglucosamine:polypeptide beta-N-acetylglucosaminyltransferase.

Using a combination of conventional and affinity chromatographic techniques, we have purified a uridine diphospho-N-acetylglucosamine:polypeptide beta-N-acetylglucosaminyltransferase (O-GlcNAc transferase) over 30,000-fold from rat liver cytosol. The transferase is soluble and very large, migrating with an apparent molecular weight of 340,000 on molecular sieve chromatography. Analysis of the purified enzyme on sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals two protein species migrating at 110 (alpha subunit) and 78 (beta subunit) kDa in approximately a two-to-one ratio. Thus, the enzyme likely exists as a heterotrimer complex with two subunits of 110 kDa and one of 78 kDa (alpha 2 beta). The alpha subunit appears to contain the enzyme's active site since it is selectively radiolabeled by a specific photoaffinity probe (4-[beta-32P]thiouridine diphosphate). Photoinactivation and photolabeling of the enzyme are dependent on time and long wavelength ultraviolet light. Photolabeling of the alpha subunit is specifically blocked by UDP. The enzyme has an extremely high affinity for UDP-GlcNAc (Km = 545 nM). This unusually high affinity for the sugar nucleotide donor probably provides the enzyme an advantage over the nucleotide transporters in the endoplasmic reticulum and Golgi apparatus which compete for available cytoplasmic UDP-GlcNAc. The multimeric state and large size of the O-GlcNAc transferase imply that its activity may be highly regulated within the cell.     

Modeling the biomechanics of the mandible: a three-dimensional finite element study.

Three-dimensional finite element models of a partially edentulated human mandible were generated to calculate the mechanical response to simulated isometric biting and mastication loads. The level of mesh refinement was established via a convergence test and showed that a model with over 30,000 degrees of freedom was required to obtain analysis accuracy. The functional loading cases included muscle loading based on an algorithm that assigns muscle forces in accordance with muscle cross-sectional area, while maintaining static equilibrium. Results were found for isometric application of unilateral and bilateral bite and mastication loading, and two different sets of displacement boundary conditions were imposed at the condyles. The mechanical response is shown in terms of displacements, principal strains, and a new measure called the 'mechanical intensity scalar'. For each load case studied, there was substantial bending in the molar region of the corpus and high tensile strains in the anterior portion of the ramus.