Cooperativity in lipid activation of 3-hydroxybutyrate dehydrogenase: role of lecithin as an essential allosteric activator

Activity of the yeast Tat2p tryptophan permease is sensitive to the anti-tumor agent 4-phenylbutyrate

4-Phenylbutyrate (PB) induces differentiation and is being intensively studied as a treatment for brain, prostate, breast, and hematopoietic cancer. While many different primary targets for PB have been proposed, the mechanism by which it causes cellular differentiation remains unknown. To identify the primary cellular target, we investigated its effects on Saccharomyces cerevisiae and showed that it inhibits tryptophan transport. We show here that PB and sorbic acid induce an ubiquitin-dependent turnover of the tryptophan permease Tat2p. However, the inhibition of transport is not a consequence of the loss of Tat2p, since it also occurs when turnover is prevented by deleting the Tat2p ubiquitination sites. When we tested the effects of PB and other growth inhibitory agents on the growth of amino acid auxotrophs, we found that several auxotrophs are hypersensitive to a number of chemically unrelated agents, including PB and some, but not all, weak acids; and this sensitivity is due to the inhibition of amino acid transport. For the inhibitory weak acids, inhibition is not confined to aromatic amino acid auxotrophs, nor is it a general weak acid stress response, since the degree of inhibition is independent of weak acid hydrophobicity and p Ka. Our results show that diverse agents affect the activity of the Tat2p permease rather than its stability and suggest the hypothesis that the anti-neoplastic action of PB is due to a decrease in the activity of surface receptors or other membrane proteins needed to maintain the transformed state.

Exposure of platelet membrane phosphatidylserine regulates blood coagulation

This article addresses the role of platelet membrane phosphatidylserine (PS) in regulating the production of thrombin, the central regulatory molecule of blood coagulation. PS is normally located on the cytoplasmic face of the resting platelet membrane but appears on the plasma-oriented surface of discrete membrane vesicles that derive from activated platelets. Thrombin, the central molecule of coagulation, is produced from prothrombin by a complex ("prothrombinase") between factor Xa and its protein cofactor (factor V(a)) that forms on platelet-derived membranes. This complex enhances the rate of activation of prothrombin to thrombin by roughly 150,000 fold relative to factor X(a) in solution. It is widely accepted that the negatively charged surface of PS-containing platelet-derived membranes is at least partly responsible for this rate enhancement, although there is not universal agreement on mechanism by which this occurs. Our efforts have led to an alternative view, namely that PS molecules bind to discrete regulatory sites on both factors X(a) and V(a) and allosterically alter their proteolytic and cofactor activities. In this view, exposure of PS on the surface of activated platelet vesicles is a key regulatory event in blood coagulation, and PS serves as a second messenger in this regulatory process. This article reviews our knowledge of the prothrombinase reaction and summarizes recent evidence leading to this alternative viewpoint. This viewpoint suggests a key role for PS both in normal hemostasis and in thrombotic disease.

Continuum solvent model studies of the interactions of an anticonvulsant drug with a lipid bilayer

Valproic acid (VPA) is a short, branched fatty acid with broad-spectrum anticonvulsant activity. It has been suggested that VPA acts directly on the plasma membrane. We calculated the free energy of interaction of VPA with a model lipid bilayer using simulated annealing and the continuum solvent model. Our calculations indicate that VPA is likely to partition into the bilayer both in its neutral and charged forms, as expected from such an amphipathic molecule. The calculations also show that VPA may migrate (flip-flop) across the membrane; according to our (theoretical) study, the most likely flip-flop path at neutral pH involves protonation of VPA pending its insertion into the lipid bilayer and deprotonation upon departure from the other side of the bilayer. Recently, the flip-flop of long fatty acids across lipid bilayers was studied using fluorescence and NMR spectroscopies. However, the measured value of the flip-flop rate appears to depend on the method used in these studies. Our calculated value of the flip-flop rate constant, 20/s, agrees with some of these studies. The limitations of the model and the implications of the study for VPA and other fatty acids are discussed.

Lipid domains in model and biological membranes

Lipid domains that occur within biological of model membranes encompass a variety of structures with very different lifetimes. The separation of membrane lipids into compositional domains can be due to lateral phase separation, immiscibility within a single phase, or interaction of lipids with integral or peripheral proteins. Lipid domains can affect the extent and rate of reactions in the membrane and provide sites for the activity of specialized proteins. Domains are likely to be involved in the process of lipid sorting to various cellular membranes, as well as in other processes which involve membrane budding or invagination.

Inactivation of D-3-hydroxybutyrate dehydrogenase by fumaroyl bis(methyl phosphate)

Fumaroyl bis(methyl phosphate) reacts with the NADH-dependent enzyme, D-3-hydroxybutyrate dehydrogenase, leading to irreversible inactivation. The bifunctional reagent cross-links the subunits of the enzyme. The inactivation is subject to saturation and protection by substrate, consistent with the reaction occurring at the active site. The stoichiometry of inactivation indicates two active sites undergo reaction with each equivalent of reagent. These results indicate that the dimeric enzyme has contiguous active sites. The reagent is likely to react with an active site lysine, consistent with previous suggestions.

Monoclonal antibodies for structure-function studies of (R)-3-hydroxybutyrate dehydrogenase, a lipid-dependent membrane-bound enzyme

Monoclonal antibodies (mAbs) have been used to study structure-function relationships of (R)-3-hydroxybutyrate dehydrogenase (BDH) (EC 1.1.1.30), a lipid-requiring mitochondrial membrane enzyme with an absolute and specific requirement for phosphatidylcholine (PC) for enzymic activity. The purified enzyme (apoBDH, devoid of phospholipid and thereby inactive) can be re-activated with preformed phospholipid vesicles containing PC or by short-chain soluble PC. Five of six mAbs cross-react with BDH from bovine heart and rat liver, including two mAbs to conformational epitopes. One mAb was found to be specific for the C-terminal sequence of BDH and served to: (1) map endopeptidase cleavage and epitope sites on BDH; and (2) demonstrate that the C-terminus is essential for the activity of BDH. Carboxypeptidase cleavage of only a few (< or = 14) C-terminal amino acids from apoBDH (as detected by the loss of C-terminal epitope for mAb 3-10A) prevents activation by either bilayer or soluble PC. Further, for BDH in bilayers containing PC, the C-terminus is protected from carboxy-peptidase cleavage, whereas in bilayers devoid of PC the C-terminus is cleaved, and subsequent activation by PC is precluded. We conclude that: (1) the C-terminus of BDH is essential for enzymic activity, consistent with the prediction, from primary sequence analysis, that the PC-binding site is in the C-terminal domain of BDH; and (2) the allosteric activation of BDH by PC in bilayers protects the C-terminus from carboxypeptidase cleavage, indicative of a PC-induced conformational change in the enzyme.