Polymer concentration effects on helix–coil transition widths

A theoretical study of effects of excluded volume intermolecular interactions on the sharpness of helix–coil transitions in solutions of polyamino acids or simple proteins indicates that the transition width may vary appreciably as a function of polymer concentration. The analysis is based on a second virial approximation for the excess free energy of mixing of a solution of polymers of varying degrees of helicity. The virial coefficients involved are roughly estimated on the basis of gross polymer geometry. For large N (degree of polymerization) the transition is found, typically to sharpen with increasing concentration, becoming second order and then first order at sufficiently high concentrations. The critical polymer concentration is found to be roughly of the order N^?1.2 ??₀^?1 for an “all or none” model and of order σ^1/2 N^?0.2 ??₀^?1 for a model with continuously variable degree of helicity (??₀ is the volume of a single helical molecule and σ^1/2 the normalized statistical weight of a helix–coil interface). In the second case for N ～ 10³ and σ ～ 10^?2–10^?4, the predicted critical concentration is in the range 10^?1–10^?3 g/cm.³ Comparison is made with experiments on solutions of poly(γ-benzyl-L glutamate).     

Vitamin B12 Status in Pregnancy among Immigrants to Britain

Haemoglobin, serum vitamin B₁₂, and serum and red cell folate levels have been measured in 322 pregnant immigrant women in London at their first booking and in a proportion at 34 weeks of gestation and postnatally. The Indian, East-African Indian, and Pakistani and Bangladeshi patients showed significantly lower initial mean serum vitamin B₁₂ levels than the European group, the levels being lower in Hindu and Sikh patients than in Moslems. The patients of West Indian, Indian, and East-African Indian origin showed significantly lower initial mean haemoglobin levels than the immigrants from European countries. Though there was no overall correlation between haemoglobin and serum vitamin B₁₂ level the incidence of hypersegmented polymorphs and macrocytosis in the peripheral blood was highest in the Indian and East-African Indian patients, and both these features were particularly frequent in patients with subnormal serum vitamin B₁₂ levels. Only one patient, however, had overt megaloblastic anaemia due to vitamin B₁₂ deficiency. The Indian patients whose red cell folate levels were less than 200 ng/ml also had a lower mean serum vitamin B₁₂ level than those with red cell folate levels greater than 200 ng/ml. The Indian patients had smaller babies than the Europeans but this was not related to the differences in vitamin B₁₂ status between the two groups. However, out of 39 babies of the Indian group 5 (13%) showed subnormal serum vitamin B₁₂ levels in the first 10 days of life, the lowest level being 120 pg/ml.Though there was an overall statistically significant fall in serum vitamin B₁₂ between first booking and 34 weeks of pregnancy there was no significant fall in serum vitamin B₁₂ in those who initially had subnormal levels. Thus many Indian women are vitamin B₁₂ deficient in pregnancy, and this is associated with morphological blood abnormalities in many cases, but megaloblastic anaemia due to this deficiency is relatively infrequent.     

Mechanism of Oncogenic Transformation by Rous Sarcoma Virus: I. Intracellular Inactivation of Cell-Transforming Ability of Rous Sarcoma Virus by 5-Bromodeoxyuridine and Light

Chick embryo fibroblasts brought into stationary phase of growth by maintenance in serum-free Eagle's MEM medium were infected with the Bryan strain of Rous sarcoma virus (B-RSV) and incubated for 18 hr in the presence of 5-bromo-deoxyuridine (BUdR). The cells were then allowed to resume growth and deoxyribonucleic acid (DNA) synthesis by addition of an enriched F12 medium containing serum and RSV antibody to prevent spread of viral infection. After 48 hr, the cultures were exposed for various periods to visible light, overlaid with solid culture medium, and observed for the appearance of foci of transformed cells. In cultures treated with BUdR at the time of infection, exposure to light resulted in a suppression of focus formation of from 50 to 90% in various experiments. Treatment with BUdR for 18 hr before infection or on the day after infection, followed by exposure to light, had no effect on focus formation. In cultures in which almost all cells were infected, treatment with BUdR followed by exposure to light did not result in cell death. This suggests that suppression of transformation is not due to selective killing of infected cells by this treatment but rather to the intracellular inactivation of the transforming ability of Rous sarcoma proviral DNA.     

Use of 3H-labeled triether, a nonabsorbable oil-phase marker, to estimate fat absorption in rats with cholestyramine-induced steatorrhea

A tritium-labeled glycerol triether was tested as a nonabsorbable oil-phase marker in studies of fat absorption in normal rats and in rats with steatorrhea induced by various doses of cholestyramine. Animals were fed a test meal containing (3)H-labeled triether and (14)C-labeled trilinolein. Fat absorption was estimated in the following three ways: (a) by isotope ratios (the change in (3)H/(14)C in the test meal and in feces); (b) by isotope recovery (the total fecal excretion of (14)C radioactivity); and (c) by chemical recovery (the total fecal fat excretion). Absorption calculated from isotope ratios agreed well with that calculated from isotope recovery over a range of fat absorption of 50-100%, thus validating the use of this lipid marker under these conditions of fat malabsorption. Absorption calculated from chemical recovery was consistently poorer than that calculated from isotope ratios or isotope recovery, thus suggesting that cholestyramine increased the excretion of nondietary (endogenous) fat. Triether may be of value for studying the absorption of compounds present predominantly in the oil phase during digestion and may have significant advantages over other proposed lipid markers.     

Proteins of the thermophilic fungus Humicola lanuginosa. II. Some physicochemical properties of a cytochrome c

A cytochrome c from Humicola lanuginosa is unique among eukaryotic cytochromes c in having phenylalanine as Residue 74. This protein has certain properties which differ from those of other cytochromes c to which it is generally similar. The Humicola cytochrome c is as stable as horse heart cytochrome c in urea, but more stable than both horse heart and yeast cytochromes c in acidic and alkaline conditions. Spectrophotometric titration of the four tyrosyl residues of the Humicola protein was nonsigmoidal with a pK_app of 11.4. Solvent perturbation difference spectra indicate that 50% of the tyrosyl residues are exposed to solvent in the native protein, and that the single tryptophanyl and all four tyrosyl residues become exposed in 8 m urea. Certain unusual features in both the optical rotatory dispersion and circular dichroism spectra in the 290-250-nm region are tentatively attributed to the substitution of phenylalanine for tyrosine at position 74.     

Affinity chromatography of thermolysin and of neutral proteases from B. subtilis

Affinity chromatographic systems are described for the purification of neutral metalloendopeptidases on columns of acetyl-$\text{D}$-phenylalanine or succinyl-$\text{D}$-leucine covalently linked to Sepharose by spacers of various lengths. The neutral proteases of $\text{B. subtilis}$ are separated in a single chromatographic procedure from all other proteins of the culture filtrates and subfractionated into two active species. An analogous chromatographic system is effective in the purification of thermolysin of $\text{B. thermoproteolyticus}$.