Showing papers by "Michael Levitt published in 1994"

Different protein sequences can give rise to highly similar folds through different stabilizing interactions.

Abstract: We report an interesting case of structural similarity between 2 small, nonhomologous proteins, the third domain of ovomucoid (ovomucoid) and the C-terminal fragment of ribosomal L7/L12 protein (CTF). The region of similarity consists of a 3-stranded beta-sheet and an alpha-helix. This region is highly similar; the corresponding elements of secondary structure share a common topology, and the RMS difference for "equivalent" C alpha atoms is 1.6 A. Surprisingly, this common structure arises from completely different sequences. For the common core, the sequence identity is less than 3%, and there is neither significant sequence similarity nor similarity in the position or orientation of conserved hydrophobic residues. This superposition raises the question of how 2 entirely different sequences can produce an identical structure. Analyzing this common region in ovomucoid revealed that it is stabilized by disulfide bonds. In contrast, the corresponding structure in CTF is stabilized in the alpha-helix by a composition of residues with high helix-forming propensities. This result suggests that different sequences and different stabilizing interactions can produce an identical structure.

Use of alveolar carbon monoxide measurements to assess red blood cell survival in hemodialysis patients

Abstract: We recently described a simple technique for measuring RBC survival based on measurements of the concentration of carbon monoxide (CO) in alveolar air, corrected for environmental CO using a device that equilibrates with atmospheric CO at the same rate as does the patient. The purpose of the present report is to demonstrate the clinical utility of this method via measurements of RBC turnover in hemodialysis patients. Prior to dialysis, the mean RBC survival of 9 chronic dialysis patients was 70 ± 9 days, about 50% shorter than that of 32 healthy subjects. During the dialysis, the endogenous Pco increased by 14% (P < 0.05) while subjects not undergoing dialysis had a 6% fall in endogenous Pco over this same time period. Thus, this technique demonstrated that the hemodialysis procedure resulted in about a 20% increase in RBC destruction. This increased RBC destruction has not been detectable with previous methodologies (including conventional measurements of CO production) due to the insensitivity and lack of reproducibility of conventional techniques. We conclude that the simple, non-invasive measurement of endogenous Pco provides the most accurate available means of assessing the influence of a variety of acute manipulations on RBC survival. © 1994 Wiley-Liss, Inc.

Study of constancy of hydrogen-consuming flora of human colon

Abstract: The constancy of the hydrogen consuming flora of the human colon was studied in 15 healthy subjects via two measurements obtained 18 to 36 months apart. Hydrogen disappearance rate and the major products of H2-consuming bacteria, methane and sulfide, were measured during incubation of fecal homogenates with excess hydrogen and sulfate. In 11/15, the hydrogen consumption rate and the predominant hydrogen-consuming pathway (methanogenesis, sulfate reduction, or neither) remained constant. However, major shifts in these pathways were observed in four subjects, with two losing and two gaining the ability to produce methane. Methanogenesis was associated with the highest hydrogen consumption rate. This study demonstrates that clinically unrecognizable, major alterations of the colonic flora occur in healthy subjects. Understanding of the factors responsible for these alterations might allow for therapeutic manipulation of the colonic flora.