Membrane lipids enzymatic action

Mode of action Activation of macrophages/monocytes release of endogenous mediators such as lipids from arachidonic acid, reduced oxygen species, proteins 1. Pore formation in cell membranes 2. Enzymatic modification of specific substrates in the cytosol of host cells (AB-type toxins) 3. Superantigen stimulation of the immune system... 

THC and CBD differ in their action on membrane electrical properties. It has also been found that THC causes large shifts in the transition temperature of membrane lipids [ 110, 111 ]. A possible relationship between this effect and microsomal demethylase activity was observed, and it was suggested that THC could influence enzymatic action by membrane effects. 

FIGURE 3.3 Enzymatic action on membrane lipids. The head groups of membrane lipids have limited mobility due to the dense packing of Hpid molecules in the membrane phase. In this case, it is the enzyme that goes to the Hpid. This phenomenon occurs on both sides of the membrane. 

Figure 11.3 The simplest RNA cell, consisting of two ribozymes (two RNA-genes provided with enzymatic activity). Rib-1 (ribosome 1) and Rib-2 (ribosome 2), whose concerted action permits shell and core replication. Rib-1 is an RNA repli-case, capable of making copies of itself and of Rib-2. Rib-2 makes the lipid membrane, converting precursor A to surfactant S. Being based on RNA replication, it is also able to evolve. (Adapted from Szostak et al., 2001 see also Luisi et al, 2002.)...

The action of phospholipase A2 on mixed monolayers of natural and polymerizable lipids can be measured under constant surface pressure by the contraction of the monolayer as a function of time as depicted schematically in Fig. 39. It turns out that the chief parameter influencing the enzymatic activity is the miscibility of the lipid components and not the fact whether the film is polymerized or not. In mixed and demixed membranes the enzyme is able to hydrolyze the natural lipid component, but with considerable differences in the hydrolizing rate (Fig. 40). A pure dilauroyllecithin (DLPC) monolayer is completely hydrolyzed in a few minutes after injecting the enzyme... 

From the earliest description of the toxin s actions on neuronal systems, it emerged that a-LTX affects specifically the presynaptic element, from which it causes massive neurotransmitter release (e.g., Longenecker et al. 1970). The toxin has no major enzymatic activities (Frontali et al. 1976). Crucially, a-LTX has been discovered to create Ca2+-permeable channels in lipid bilayers (Finkelstein et al. 1976), and a large body of evidence shows that Ca2+ influx through membrane channels induced by a-LTX in the presynaptic membrane accounts for a major part of its effect. Pore formation occurs in all the biological systems mentioned above, but the features of a-LTX-triggered release cannot be fully explained by the toxin pore. 

The relative content of the dietary fat components varies with different sources but generally the physico-chemical properties are rather similar. For absorption to take place the physico-chemical properties of the fat have to be changed. This takes place as a consequence of the lipolytic activity in the intestinal tract and the addition of bile to chyme. Through lipolytic enzymes the dietary lipids are converted to more polar products. Bile contributes bile salt-phospholipid-cholesterol aggregates to the intestinal content (cf. Chapter 13). The concerted action of these agents is the formation of lipid products in a physical state which allows them to be transported into the enterocyte membrane and onwards for further metabolism in the cell. Bile salts are involved in the proper function of some of these enzymatic reactions and in the formation of product phases on which a normal uptake process is based. Little is known at present of the importance of bile salts for the intracellular reactions following uptake of fat into the enterocyte. Different aspects of intestinal lipid absorption have been reviewed in recent years by Patton [7], Thomson and Dietschy [8], Carey [9], Carey et al. [10], Wells and Direnzo [11], and Grundy [12]. The role of bile acids in fat absorption has been discussed by Holt [13]. 

It is interesting to note that, historically, protein played the favorite role in attempts to understand the functional dynamics of cellular metabolism and membrane physiology. Once the functional importance of the lipid component of membranes was recognized, most of the initial experimental effort was aimed at hydrophobic effects. However, it is now recognized that surface charge effects associated with membrane phospholipids have been shown to profoundly affect enzymatic activity and influence the action of various hormones, vitamins, and drugs. 

The cellular uptake and release of glucose and other monosaccharides is a protein-mediated process. Such proteins are embedded into the cell membrane and can be regarded as hydrophilic pores within the hydrophobic lipid bilayer. Monosaccharide transporters are substrate specific and stereospecific their action is saturable and can be inhibited by specific competitors. Like an enzymatic reaction, the kinetic characteristics can be described by affinity constants and the number of transporter protein molecules determines maximal transport velocity (Vmax)-... 

Compounds 292-295 act as inhibitors of protoporphyrinogen oxidase (Protox) - an enzyme in the chloroplasts of the plant cells that oxidizes protoporphyrinogen IX (303) to produce protoporphyrin IX (304) (Scheme 66) [261], In turn, 304 is a precursor molecule for both chlorophyll and heme. When protoporphyrinogen oxidase is inhibited, protoporphyrinogen IX is accumulated and transferred from chloroplasts into the cytoplasm, where non-enzymatic conversion of 303 to 304 occurs. When present in cytoplasm, 304 is cytotoxic due to interaction with oxygen upon action of light, which results in formation of singlet O2 molecules. O2 causes lipid peroxidation, membrane disruption and plant cell death. 

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