Some biochemical threshold mechanisms

Certain arrangements of enzymatic (bimolecular) subsystems lead to characteristic threshold-type response. Two simple cases of such systems are studied here in terms of steady state behavior and explicit relationships between system and curve parameters. It is found that the curvature of the threshold curve is directly related to the equivalent Michaelis constant and, in the case of saturated threshold curve, the slope of the curve at the idealized threshold is limited by the ratio of saturation to threshold. This slope may be appreciably increased up to a stepwise response at the threshold if a multisubstrate complex of the enzyme is the only species which affects the enzyme mediated transport.     

Some biochemical aspects of N-nitroso compounds

Contents. I. Introduction. II. Occurrence and carcinogenicity. III. Chemistry. A. Formation. B. Catalysis and inhibition. C. Some aspects of chemical reactivity. D. Detection and determination. IV. Metabolism and carcinogenic action. V. Therapeutic 2-haloethylnitrosoureas.     

Molecular mechanisms of olfactory reception IV. Some biochemical characteristics of the camphor receptor from rat olfactory epithelium

Some parameters of the receptor element from the rat olfactory epithelium are evaluated; it is characterized by a high affinity for camphor (K_D = 1.5 · 10^?9 M). Triton X-100 has no marked effect on the binding of [³H]camphor. Neither RNAase nor phospholipase C affected [³H]camphor-binding activity. Pronase and trypsin abolished [³H]camphor binding activity by 65 and 40%, respectively. Sulfhydryl reagents decrease the binding of [³H]camphor by a factor of 5–8. The isoelectric point of the receptor solubilized with Triton X-100 is 4.8, as determined by isoelectric focusing. The molecular weight of the receptor as determined by gel electrophoresis is about 120 000. It is proposed that the camphor receptor is a membrane protein containing sulfhydryl groups and playing a key role in olfactory reception.     

Developmental mechanisms of threshold evolution in a polyphenic beetle

Polyphenic development is thought to play a pivotal role in the origin of morphological novelties. However, little is known about how polyphenisms evolve in natural populations, the developmental mechanisms that may mediate such evolution, and the consequences of such modification for patterns of morphological variation. Here we examine the developmental mechanisms of polyphenism evolution in highly divergent natural populations of the dung beetle, Onthophagus taurus. Males of this species express two alternative morphologies in response to larval feeding conditions. Favorable conditions cause males to grow larger than a threshold body size and to develop a pair of horns on their heads. Males that encounter relatively poor conditions during larval life do not reach this threshold size and remain hornless. Exotic populations of O. taurus have diverged dramatically in body size thresholds in less than 40 years since introduction to new habitats, resulting in the expression of highly divergent and novel horn length-body size scaling relationships in these populations. Here we show that larvae of populations that have evolved a larger threshold body size (1) have to accumulate greater mass to become competent to express the horned morph, (2) require more time to complete the final instar, (3) are less sensitive to the juvenile hormone (JH) analogue methoprene, and (4) exhibit a delay in the sensitive period for methoprene relative to other developmental events. JH has been shown previously to control horn expression in this species. Our results show that threshold evolution may be mediated via changes in the degree and timing of sensitivity to JH and may result in correlated changes in the dynamics and duration of larval development. Strain-specific differences in JH sensitivity have previously been demonstrated in other insects. However, to the best of our knowledge this is the first demonstration that changes in the timing of the sensitive period for JH may play an equally important role in the evolution of novel thresholds. We discuss our findings in the context of the developmental regulatory mechanisms that underlie polyphenic development and use our results to explore the consequences of, and constraints on, polyphenism evolution in nature.     

The biochemical mechanisms of action of the interferons

The interferon family of proteins consists of a variety of antigenically distinct types that are encoded by different genes. Several of these genes have been cloned and the availability of large amounts of the recombinant interferons has greatly facilitated characterization of their biochemical and biological activities. The specificity of interaction of interferons with cell surface receptors has been investigated in binding and covalent cross-linking experiments employing 125I-labeled interferons. In addition to their antiviral activity, the interferons have effects on cell growth and differentiation. The interferons also are potent modulators of the immune response. The interferons exert their biological activities by altering the expression of several cellular proteins. Current knowledge about the mechanisms of signal transduction and the regulation of expression of interferon-induced mRNAs and proteins are discussed.     

Some biochemical properties of swine uterus carnosinase

Carnosinase of swine uterus reacts strongly to Mn2+ ions with an increase of the activity: in the presence of 0.25 MnCl2 the activity increases over 5-fold, while at 1 and 2 mM--the increase is 8- and 10-fold respectively. The enzyme is characterized by low stability during storage, especially in the presence of manganese ions. Kinetic properties of uterus carnosinase change depending on a phase of the oestrous cycle of the sow. In the peak luteal phase (5th-13th day of the cycle) Km values were twice as high as in the follicular phase (zero day--beginning of the rut, and 19th day--preoestrus). Two molecular forms of carnosinase were found in the extracts from uterus in the luteal phase of the oestrous cycle, analysed with the method of Sephadex G-100 gel filtration. These were A and B forms, with predominating content of the latter form. This form was characterised by a 2-fold higher Km value compared to the form A.