Partial hydrodynamic screening of confined linear and circular double-stranded DNA dynamics.

We performed experiments and simulations to investigate the influence of hydrodynamic interaction on the diffusion dynamics of circular and linear λ-DNA confined in nanoslits. Contrary to the common assumption that intrachain hydrodynamic interaction (HI) is completely screened when polymers are confined in channels with height h smaller than the radius of gyration R(g), it is found that the HI is partially screened and approaches complete screening only for R(g)≪h. For λ-DNA, the HI becomes nearly completely screened only when the channel height is smaller than the Kuhn length. In addition, the dynamics of linear and circular λ-DNA in very strong confinement is shown to be independent of the chain topology.

Factors affecting growth factor activity in goat milk.

Growth factors that are present in goat milk may be responsible for its beneficial effects on the digestive system as described in ancient Chinese medical texts. To develop a nutraceutical product rich in growth factors for promoting gastrointestinal health, it is essential to collect milk with consistently high growth factor activity. Therefore, we investigated the factors affecting growth factor activity in goat milk. Among the 5 breeds of dairy goats tested, milk from Nubian goats had the highest growth factor activity. Tight-junction leakage induced by a 24-h milking interval did not increase growth factor activity in the milk. Milk collected from pregnant does had a significantly higher growth factor activity than milk collected postpartum. Growth factor activity decreased during the first 8 wk of lactation, fluctuated thereafter, and then increased dramatically after natural mating. During wk 1 to 8, growth factor activity was inversely correlated with milk yield and week of lactation. No correlation was observed during wk 9 to 29. After natural mating of the goats, the growth factor activity in the milk correlated significantly with somatic cell count and conductivity (a measure of membrane permeability), and correlated inversely with milk yield. Based on the above data, goat milk with higher growth factor activity could be selectively collected from Nubian pregnant does.

Effect of D57N mutation on membrane activity and molecular unfolding of cobra cardiotoxin.

Cobra cardiotoxins (CTXs) are able to adopt a three-fingered beta-strand structure with continuous hydrophobic patch that is capable of interacting with zwitterionic phospholipid bilayer. In addition to the four disulfide bonds that form the rigid core of CTXs, Asp57 near the C-terminus interacts electrostatically with Lys2 near the N-terminus (Chiang et al. 1996. Biochemistry. 35:9177-9186). We indicate herein, using circular dichroism and the time-resolved polarized tryptophan fluorescence measurement, that Asp57 to Asn57 (D57N) mutation perturbs the structure of CTX molecules at neutral pH. The structural stability of the D57N mutant was found to be lower, as evidenced by the reduced effective concentration of the 2,2,2-trifluoethanol (TFE)-induced beta-sheet to alpha-helix transition. Interestingly, the single mutation also allows a greater degree of molecular unfolding, because the rotational correlation time of the TFE-induced unfolding intermediate is larger for the D57N mutant. It is suggested that the electrostatic interaction between N- and C-termini also contributes to the formation of the functionally important continuous hydrophobic stretch on the distant end of CTX molecules, because both the binding to anilinonaphthalene fluorescent probe and the interaction with phospholipid bilayer were also reduced for D57N mutant. The result emphasizes the importance of the hydrophobic amino acid residues near the tip of loop 3 as a continuous part of the three-fingered beta-strand CTX molecule and indicates how a distant electrostatic interaction might be involved. It is also implicated that electrostatic interaction plays a role in expanding the radius of gyration of the folding/unfolding intermediate of proteins.

Restoration of juvenile baseline growth hormone secretion with preservation of the ultradian growth hormone rhythm by continuous delivery of growth hormone-releasing factor.

The ability of continuously delivered GH-releasing factor (GRF) to enhance GH secretion while maintaining the normal ultradian GH rhythm was investigated. Synthetic human GH-releasing factor (hGRF(1-44)NH2) was continuously infused for 4 days by means of i.v. catheters to 11-week-old broiler chickens. At this age, overall endogenous GH secretion is low, and baseline GH is barely detectable. Six birds per treatment received vehicle (control), 0.324 mg hGRF(1-44)NH2/kg body weight per day (low dose) or 3.24 mg hGRF(1-44)NH2/kg body weight per day (high dose). After 4 days of GRF conditioning, concurrent with continued GRF infusion, serial blood samples were removed via atrial catheters at 15-min intervals for 6 h and GH plasma profiles determined. High dose GRF significantly increased GH plasma concentrations over tenfold compared with controls; however, most of this increase reflected an increase in basal GH, which was reinstated to juvenile baseline levels. Augmentation of pulse amplitude above this increased baseline was not proportionately as high, and failed to reach juvenile levels. The ultradian rhythm of GH was not altered by continuous GRF administration. Both low and high dose GRF treatments resulted in significant enlargement of the anterior pituitary gland. Total pituitary GH mRNA levels, although elevated over twofold by GRF treatment, were not significantly different from controls. Measures of plasma GH magnitude (overall and baseline mean, and peak amplitude) were significantly correlated with pituitary GH mRNA for control birds, but were not correlated for GRF treatments. Feed intake was markedly depressed (33%) on the high dose GRF treatment, in conjunction with total inhibition of body weight gain over the 4-day period of administration. Longitudinal bone growth and width of the epiphyseal growth plate were also significantly reduced by high dose GRF treatment, probably reflecting the reduced level of nutrient intake, despite high circulating concentrations of GH.