Individual variability and population regulation: an individual-based model

To study the influence of individual variability on population dynamics an individual-based model of the dynamics of a single population consisting of different individuals is constructed. The model is based on differences in individual assimilation rates due to intraspecific competition and variability of initial weights. The model exhibits "imperfect regulation", i.e., the number of individuals in the population oscillates and sooner or later the population becomes extinct. When individual variability is included, the model produces longer population extinction times than without individual variability. The average extinction time is not however a monotonic function of the degree of individual variability.     

A colostral protein that induces the growth and differentiation of resting B lymphocytes

We describe the first protein of mammalian origin that induces the growth and differentiation of resting B lymphocytes. A proline-rich protein has been isolated from sheep colostrum. A purified proline-rich protein preparation (PRPP) induced resting mouse B cells into and supported their progression through the cell cycle at frequencies comparable with those seen for LPS. Differentiation of resting B cells to plaque formation was also supported as efficiently by PRPP as it was by LPS. However, PRPP was distinct from LPS in that it supported the growth and differentiation of resting B cells derived from either C3H/Tif or C3H/HeJ mice. Splenocytes from neonatal mice responded robustly to PRPP with the growth and differentiation of contained B cells to plaque formation. Unlike LPS, PRPP did not induce detectable Ig isotype switching.     

Production of cellulases and xylanase by Trichoderma reesei F-522 on pretreated wheat straw

Autohydrolyzed and ethanol-alkali pulped wheat straw was examined as a candidate feedstock for both cellulase and xylanase production and enzymatic hydrolysis. Submerged cultures of Trichoderma reesei F-522 grown on hydrothermally modified straw provided culture supernatants of the highest enzymatic activities, whereas the maximal efficiency of enzymatic hydrolysis was recorded in straw treated with ethanol-NaOH mixture. Some culture conditions were optimized to improve the growth and cellulase production by T. reesei on autohydrolyzed wheat straw.     

The enzymatic hydrolysis and fermentation of pretreated wheat straw to ethanol

Autohydrolysis and ethanol-alkali pulping were used as pretreatment methods of wheat straw for its subsequent saccharification by Trichoderma reesei cellulase. The basic hydrolysis parameters, i.e., reaction time, pH, temperature, and enzyme and substrate concentration, were optimized to maximize sugar yields from ethanol-alkali modified straw. Thus, a 93% conversion of 2.5% straw material to sugar syrup containing 73% glucose was reached in 48 h using 40 filter paper units/g hydrolyzed substrate. The pretreated wheat straw was then fermented to ethanol at 43 degrees C in the simultaneous saccharification and fermentation (SSF) process using T. reesei cellulase and Kluyveromyces fragilis cells. From 10% (w/v) of chemically treated straw (dry matter), 2.4% (w/v) ethanol was obtained after 48 h. When the T. reesei cellulase system was supplemented with beta-glucosidase from Aspergillus niger, the ethanol yield in the SSF process increased to 3% (w/v) and the reaction time was shortened to 24 h.     

Mathematical Model of Leukemic-Cell Circulation in the Mouse

L1210 leukemic cells injected in vivo are eliminated from the blood and disintegrated in organs such as the lungs and liver. We present a compartmental model which reproduces one type of in vivo experiment, based on the so-called perfusion curves. Although the data are not complete and some are only approximated, modeling gives a consistent picture of the process.     

Interconversion of the salicylic acid signal and its glucoside in tobacco

Salicylic acid (SA) has been proposed to play a role in the induction of pathogenesis-related (PR) proteins and systemic acquired resistance (SAR) in tobacco. Since SA is rapidly converted to salicylic acid β-glucoside (SAG) in tobacco, we have attempted to assess the role of SAG in pathogenesis by application of chemically synthesized SAG to tobacco leaves. SAG was as active as SA in induction of PR-1 gene expression. This induction was preceded by a transient release of SA, which occurred in the extracellular spaces. The existence of a mechanism that releases SA from SAG suggests a possible role for SAG in SAR.     

D-amino acid levels in human physiological fluids

Plasma, urine, cerebrospinal fluid (CSF), and amniotic fluid were examined to determine whether free D-amino acids were present and if so at what levels. It was found that D-amino acids exist in all physiological fluids tested, but that their level varied, considerably. The lowest levels of D-amino acids were usually found in amniotic fluid or CSF (almost always <1% of the corresponding L-amino acid). The highest levels were found in urine (usually tenth percent to low percent levels). Pipecolic acid seemed to be different from the other amino acids tested in that it was excreted primarily as the D-enantiomer (often >90%). Correspondingly high levels of D-pipecolic acid were not found in plasma. Some of the trends found in this work seemed to be analogous to those found in a recent rodent study. © 1993 Wiley-Liss, Inc.     

Conformational mimicry: Synthesis and solution conformation of a cyclic somatostatin hexapeptide containing a tetrazole cis amide bond surrogate

Potent, cyclic hexapeptide analogues of somatostatin are generally believed to adopt some common secondary structural features: a II′ β turn at one end of the cycle, and a type VI turn with a cis amide bond at the other. A proposed cis amide surrogate, the 1,5-disubstituted tetrazole, has been placed into a cyclic hexapeptide analog of somatostatin in order to constrain the putative cis amide bond. The final cyclization was done by either chemical or enzymatic means. The product, cyclo(Ala⁶-Tyr⁷-D -Trp⁸-Lys⁹-Val¹⁰-Phe¹¹-Ψ[CN₄]), was found to have 83% of the activity of somatostatin. Solution nmr analysis in DMSO/water revealed that the backbone as well as side chain χ₁ and χ₂ were well ordered. Relaxation matrix methods were used to extract distance restraints from the nuclear Overhauser effect spectroscopy data set, and these were used in a systematic search of torsional space to identify structures consistent with the nmr data. Restrained minimizations of these structures using a number of different force fields produced structures having the expected βII′ turn at D -Trp⁸-Lys⁹ and αβVIa turn in the Phe¹¹-Ψ[CN₄]-Ala⁶ portion of the molecule. The similarity of the minimized structures to those previously reported for cyclic hexapeptide analogues of somatostatin confirms the similarity of the tetrazole geometry to that of the cis amide in solution. © 1995 John Wiley & Sons, Inc.