Showing papers by "Michael Levitt published in 1972"

Use of pulmonary hydrogen (H 2 ) measurements to quantitate carbohydrate absorption: Study of partially gastrectomized patients

Abstract: A technique was developed to quantitate the absorption of ingested carbohydrate by means of continuous measurements of pulmonary H(2) excretion. This technique is based on the observation that H(2) is produced in the colon when carbohydrate is fermented by colonic bacteria, and this H(2) is then excreted by the lungs. The quantitative relationship of pulmonary H(2) excretion to unabsorbed carbohydrate was studied in nine subjects. After ingestion of 6.5, 13, and 26 g of lactulose (a nonabsorbable disaccharide), H(2) excretion increased linearly, averaging (+/-1 SEM) 13+/-3.5, 23+/-7.2, and 49+/-7 ml per 2 hr. Because of consistent individual differences in H(2) excretion per gram of lactulose, the variability of this linear response was less in a given subject, with the H(2) excretion after 6.5 g and 26 g lactulose dosages averaging 55+/-4.2% and 214+/-16% of that observed after the 13 g dose. It was further demonstrated with fecal homogenates, as well as in rats after direct intracecal instillation of carbohydrate, that there was no significant difference in the rate of H(2) formation from lactulose as compared with the normally ingested sugars. Thus, a subject's H(2) excretion after a 13 g dose of lactulose can be used as a standard to convert H(2) excretion after ingestion of other carbohydrates into grams of carbohydrate not absorbed. Application of this technique to seven partially gastrectomized patients indicated all subjects malabsorbed a portion of a 100 g dose of glucose whereas six of seven completely absorbed a 25 g dose. Malabsorption of physiologic quantities of various carbohydrates was clearly demonstrated in one subject. This technique appears to provide quantitative information on carbohydrate malabsorption not readily obtained by presently available techniques.

Simultaneous study of the metabolic turnover and renal excretion of salivary amylase- 125 I and pancreatic amylase- 131 I in the baboon.

Abstract: The metabolic turnover of salivary and pancreatic amylase was studied in the baboon, an animal with a serum amylase level and renal clearance of amylase similar to man. Purified amylase was electrolytically iodinated. Although iodinated and uniodinated amylase had similar gel filtration, electrophoretic, enzymatic, glycogen precipitation characteristics, the labeled enzyme was cleared less rapidly by the kidney than was the unlabeled material. However, urinary iodinated amylase which had been biologically screened by the kidney had a renal clearance and serum disappearance rate indistinguishable from unlabeled amylase and thus can serve as a tracer in metabolic turnover studies. Administration of a mixture of salivary amylase-(125)I and pancreatic amylase-(131)I made it possible to simultaneously measure the serum disappearance and renal clearance of these two isoenzymes. The metabolic clearance of both isoenzymes was extremely rapid with half-times of about 130 min. This rapid turnover of serum amylase probably accounts for the transient nature of serum amylase elevation which frequently occurs in pancreatitis. Pancreatic amylase-(131)I was consistently cleared more rapidly (mean clearance ratio: 1.8) by the kidney than was salivary amylase-(125)I. This more rapid renal clearance of pancreatic amylase may help to explain the disproportionate elevation of urinary amylase relative to serum amylase observed in pancreatitis.

Floating Stools — Flatus versus Fat

Abstract: Factors influencing the density of stools from 33 healthy subjects (nine had floating and 24 sinking stools) and six patients with steatorrhea were investigated. All floating stools sank when their gas volume was compressed by positive pressure. Thus, to float, stools must contain gas. After degassing, previously floating and sinking stools had similar specific gravities, indicating that the floating or sinking propensity of such stools depends upon differences in gas rather than fat content. A high stool gas content (and hence a floating stool) in healthy subjects appeared related to colonic methane production. Steatorrheic stools had a relatively normal gas content. The density of their nongaseous fraction was less than normal, but resulted primarily from increased water rather than fat content. Thus, stools float because of an increased content of gas or water (or both); the floating stool should not be considered a sign of steatorrhea.

Folding of Nucleic Acids

Abstract: This paper describes and discusses nucleic acid conformations, the energy contributions that stabilize them, and the ways the stable conformations are formed. Preferred conformations of nucleotides and of double, triple and super helices in nucleic acids are described briefly. Next the enthalpy and entropy of nucleic acid order/disorder transitions are reviewed. It is concluded that (a) the double helix is stabilized primarily by interbase hydrogen bonds, and (h) the available thermodynamic parameters for single-strand stacking are probably incorrect. The conformation of single strands in solution is discussed briefly. The kinetics of helix formation is described; the transition state of three base pairs that has been proposed supports the idea that the strength of hydrogen bonds depends on the access of water molecules to the base pair. Hairpin formation and more complex secondary structures of nucleic acids are described and explained in terms of rate of formation, stability, and control of secondary structure. Transfer RNA has tertiary structure as well as secondary structure. Schematic diagrams are used to show how experimental evidence fixes different parts of the tRNA tertiary structure. The detailed interactions in the proposed model are supported by the sequence of cell-wall glycine tRNA, which can be charged with an amino acid without participating in protein synthesis. Sequential steps in the folding and unfolding of the proposed tRNA conformation are outlined. Finally, the abilities of nucleic acids and proteins to form stable tertiary structures are compared and the evolutionary consequences are considered. Nucleic acids and proteins have been studied intensively over the past 25 years. Although similar techniques have been used on each of these important classes of macromolecule, available information for each is very different. X-ray diffraction methods have elucidated conformations of both nucleic acids and proteins but, whereas the known conformations of different proteins are very different, those of nucleic acids are all simple variants of the Watson-Crick double helix. It is easier, however, to study nucleic acid conformations in