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1. 1. Liver microsomes were assayed fluorometrically after pretreatment of rats with 3-methylcholanthrene, chrysene and phenobarbital. A difference in the fluorescence was obtained after pretreatment with phenobarbital when ANS fluorescence was combined with variable temperature. 2. 2. UDP Glucuronosyltransferase was solubilized after different pretreatments of the rats using trypsin digestion and digitonin solubilization. It was possible to get more enzyme into solution using 3-methylcholanthrene and chrysene as the inducer. Digitonin did not release as much enzyme after phenobarbital treatment of rats. 3. 3. Differently-acting inducers make changes in the protein-lipid interaction in the membranes. These changes are reflected also in the enzyme activities under in vitro conditions.

1. 1. The specific interaction of bovine myelin A1 basic protein with lipids at the air-water interface was studied. The interaction was measured by recording the changes in surface pressure and surface radioactivity using 131I-labelled A1 basic protein. The highest affinity of the A1 basic protein was found for cerebroside sulphate, a lipid which is characteristic for the myelin lipids. 2. 2. The proteolytic degradation of the A1 basic-protein-lipid complex by proteolytic enzymes such as trypsin, chymotrypsin A4, subtilopeptidase A and pronase E, showed that specific regions of the protein molecule are protected after the interactions with lipids. 3. 3. The A1 basic protein-cerebroside sulphate complex was collected from the interface after tryptic hydrolysis. Peptide maps showed that the N-terminal part of the protein molecule (positions 20–113) is preserved in the lipid phase. A schematic model of the A1 basic protein lipid interaction is presented.

A variety of techniques, both biochemical and physical, have shed light upon the structure of biological membranes. A consensus of opinion has developed by which the membrane is thought to consist of a phospholipid bilayer interspersed with proteins. Excellent reviews on this subject have recently been published. It is interesting to note that the current membrane model accentuates the fluidity of the membrane. Phospholipids undergo a rapid lateral diffusion within the two monolayers of the bilayer. Membrane proteins are also thought to diffuse freely in the lipid matrix. Which phospholipids and proteins in the membrane are in this state of motion, and how the movement of phospholipids and proteins is interrelated, are the subjects of intensive research.

There is now a vast amount of evidence to show that virtually all the multi-enzyme systems which exist in cells contain phospholipids as an integral part of their structure and activity. Generally this evidence is as follows — the enzymic activity can be substantially reduced by extracting the complexes with organic solvents e.g. aqueous acetone or by treating them with phospholipases (A or C) and largely restored by reacting them with aqueous suspensions of isolated phospholipids. Often there is little specificity about the structure of the phospholipid required although it has recently been claimed that phosphatidylserine is specific for the restoration of delipidated transport ATPase (Wheeler & Whittam, 1970). There is little evidence to show precisely why the phospholipids are essential components of such enzyme complexes. Our lack of knowledge is usually covered by all enveloping general statements, such as that they act as cement substances holding the individual enzymes with their active centres orientated towards one another so that the lipid provides a medium for electron flow within complexes and between complexes. However, these explanations would not necessarily apply to the requirement for lipids by individual particulate enzyme reactions e.g. lecithin as a co-factor for D-3-hydroxybutyrate-NAD oxidoreductase (Jurtshuk, Sekuzu & Green, 1963) or acidic phospholipids for protoheme ferrolyase (Sawada, Takeshita, Sugita & Yoneyama, 1969) and here it is possible that the phospholipid may produce some activating conformation change of the enzyme protein.

Publisher Summary This chapter discusses the extraction of water-soluble enzymes and proteins from membranes. Two classes of water-soluble proteins have been isolated from membranes. The first of these includes those proteins and enzymes which are present in the space which the membrane surrounds, but are not true components of the membrane itself. The second class of proteins and enzymes are intrinsic components of the membrane which, when separated from their normal lipid environment, acquire the characteristics of classical water-soluble proteins. The chapter discusses several chemical and physical methods employed for the extraction of water-soluble enzymes and proteins from membranes. Exposure to sonic vibrations has proved to be an effective means of disrupting cells and subcellular membranes and also has been widely used as an aid in solubilizing membrane proteins. The efficiency of solubilization of membrane proteins by sonic oscillations is influenced by the power output of the instrument, the duration of exposure and the volume of material processed. Mechanical tissue and cell homogenizers have not been widely used to solubilize membrane proteins. Proteins such as cytochrome c may be associated with membrane components such as protein or phospholipid through electrostatic interactions.

There is increasing evidence at present time that about half of the proteins of various biomembrane-types are bound to the lipids within the membrane by predominantly hydrophobic interaction (1). But almost nothing is Known about what sequences or regions of the membrane polypeptides do bind to the acyl chains of the lipids.

One approach to the study of membrane arrangement is to try to tease away membrane components and observe the effect of such treatment on ultrastructure. We found earlier that lipid can be removed from the mitochondrial inner membranes with retention of the characteristic trilaminar appearance of this membrane (1). This observation meant that the simple Dayson-Danielli model of the membrane (2) i.e., a bilayer of phospholipid sandwiched by protein could not be correct for this membrane. If so, the protein ends should collapse or become irregular when the supporting central layer was removed. However, the trilayer was retained in the absence of lipid (1). To salvage the Dayson-Danielli model supporters of this hypothesis must at least postulate that protein cross-links are present to buttress the structure.

Publisher Summary This chapter provides an overview of a panel discussion on membranes. The focus of this discussion was on the localization, turnover, and function of the protein and glycoprotein components of the membrane. It was agreed that membrane lipids are arranged in a micellar organization that corresponds to the lipid bilayer of the earlier Davson–Danielli model. However, the lipid bilayer is no longer visualized as a continuous layer; it is also not overlayed with a film of protein. Rather, an undetermined number of proteins, thought to represent a small fraction of the total membrane protein, traverse the membrane bilayer. The traversing proteins are arranged with the N terminus at the external surface, the C terminal ends of proteins extending to the cytoplasmic surface. The bulk of the membrane proteins are globular proteins roughly half-embedded or inserted into the lipid bilayer, but free to move about. The membrane proteins are heterogeneous, diverse in chemical and biochemical characteristics, and vary with the cell type. Under a variety of conditions, the individual proteins and carbohydrate residues of certain glycoproteins are shown to be in a rapid state of flux, the content of individual proteins also undergoing a continual change.