High-resolution crystal structure of deoxy hemoglobin complexed with a potent allosteric effector.

The crystal structure of human deoxy hemoglobin (Hb) complexed with a potent allosteric effector (2-[4-[[(3,5-dimethylanilino)carbonyl]methyl]phenoxy]-2-methylpropionic acid) = RSR-13) is reported at 1.85 A resolution. Analysis of the hemoglobin:effector complex indicates that two of these molecules bind to the central water cavity of deoxy Hb in a symmetrical fashion, and that each constrains the protein by engaging in hydrogen bonding and hydrophobic interactions with three of its four subunits. Interestingly, we also find that water-mediated interactions between the bound effectors and the protein make significant contributions to the overall binding. Physiologically, the interaction of RSR-13 with Hb results in increased oxygen delivery to peripheral tissues. Thus, this compound has potential therapeutic application in the treatment of hypoxia, ischemia, and trauma-related blood loss. Currently, RSR-13 is in phase III clinical trials as a radiosensitizing agent in the treatment of brain tumors. A detailed structural analysis of this compound complexed with deoxy Hb has important implications for the rational design of future analogs.

Synthesis and structure-activity relationships of chiral allosteric modifiers of hemoglobin.

A series of allosteric effectors of hemoglobin, 2-(aryloxy)-2-alkanoic acids, was prepared to investigate the effect of the stereocenter on allosteric activity. The chiral analogues were based on the lead compound, RSR13 (3b), with different alkyl/alkanoic and cycloalkyl/cycloalkanoic groups positioned at the acidic chiral center. Of the 23 racemic molecules synthesized, 5 were selected for resolution based on structure-activity relationships. One chiral analogue, (-)-(1R,2R)-1-[4-[[(3, 5-dimethylanilino)carbonyl]methyl]phenoxy]-2-methylcyclopentane carbox ylic acid (11), exhibited greater in vitro activity in hemoglobin solutions than its antipode, racemate, and RSR13. Compound (-)-(1R, 2R)-11 was equipotent with RSR13 in whole blood, is a candidate for in vivo animal studies, and if efficacious and safe has a potential for use in humans. In general, it was found that chirality affects allosteric effector activity with measurable differences observed between enantiomers and the racemates.

Intrinsic activity at the molecular level: E. J. Ariëns' concept visualized.

The concept of using affinity and intrinsic activity to analyze drug interactions with receptors has had a long history in pharmacological studies. In the simplest case, the biological response will be proportional to the amount of drug bound, i.e. its affinity. However, the biological response is also mediated by the ability of a drug when bound to exert its maximum effectiveness. This effectiveness is termed the intrinsic activity. Physicochemical processes have been thought to be at the basis of intrinsic activity. Detailed oxygen and solution binding experiments combined with X-ray crystallographic studies on allosteric effectors to hemoglobin demonstrate that these potential drug agents bind at the same site in hemoglobin with similar binding constants yet shift the allosteric equilibrium and the oxygen affinity of the T-structure by different degrees. Therefore some of the effectors with similar binding affinities for the same site exhibit varying degrees of affectiveness, i.e. they possess different intrinsic activities. The intrinsic activity of the effector is defined as the ratio of the oxygen affinity constant to the T-state with drug/oxygen affinity constant to the T-state without drug (KT+drug)/(KT control). The source of the intrinsic activity appears to be the ability of the effectors to interact with key residues such as Lys99 alpha at the binding site. These results suggest a general molecular mechanism for allosteric effector modulation of hemoglobin function that might be of use in other allosteric enzyme systems.

Synthesis and secretion of phosphorylated growth hormone by rat pituitary glands in vitro.

Rat anterior pituitary glands were incubated in buffered medium, pH 7.4, containing 32Pi. After incubation the tissue and medium were separated and a post-mitochondrial supernate (PMS) of the tissue homogenate was prepared. Gel filtration of the PMS and medium resulted in a radioactive peak which coincided with the elution volume of authentic rat growth hormone (rGH). Polyacrylamide gel electrophoresis of the radioactive peak under denaturing condition resulted in a protein-staining band having the same mobility as authentic rGH. Autoradiography of the gels revealed radioactivity precisely at the position of growth hormone as well as elsewhere. The specific radioactivity of the PMS [32P]GH was estimated to be 5 to 10 times greater than that of tissue [32P]GH. These results indicate that phosphorylated GH is synthesized and secreted by pituitary glands in vitro.