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Evidence is provided that enzymes absorb to cellular structures in a wide range of tissues . In particular, the interactions between glycolytic enzymes and the microfilaments of the cytoplasm are described . The relevance of these interactions to the compartmentation of carbohydrate metabolism is discussed . Examples are given of the variations in degree of binding during alteration of tissue metabolism and, for individual glycolytic enzymes, during fetal development and differentiation . Overall, these data support the concept that metabolic activities in the cytoplasm have an organized structure . just as the structural elements of the cytosolic compartment have evolved with the capacity to assemble and disassemble in response to the changing requirements of the organism, so the metabolic elements appear to have evolved a parallel system that provides for the appropriate positioning of an energyproducing sequence in relation to the specific, dynamic requirements of the cytoskeleton .In other papers in this supplement there are many fascinating examples of the structural role of the cytomatrix and of the phenomena of directed motion in relation to this complex spatial organization. Cell shape, motility, cytoplasmic streaming, organelle distribution, cell division, and differentiation have all been viewed from this perspective, and the data leave little doubt as to the extraordinarily broad involvement of the cytomatrical structure (16) . At a functional level, the intricate choreography of structural reorganization during these cellular processes clearly requires an appropriate energy source, one with several special features. In this paper I describe the characteristics of the interactions between the glycolytic enzymes and the components of the cytomatrix, characteristics that may be of particular relevance in this context . Enzyme Binding and CompartmentationRecent studies (11, 12, 18) on intermediary metabolism have focused on the importance of compartmentation in the regulation of cellular processes and, to quote a recent treatise on this topic . ... . . it has become overtly clear that, in biology, order in metabolism is generated by the introduction of inhomogeneity, i.e., by compartmentation" (18).To most cell biologists, the most familiar examples of the metabolic advantages of the segregation of enzymes and metabolites are associated with compartmentation of cells and subcellular organelles, where a particular metabolic organization is surrounded and separated from other metabolic compartments by a physical permeability barrier, such as a 2225 membrane. Although this type of spatial compartmentation is undoubtedly the most intensively studied at this time, it is important to stress that effective metabolic compartmentation may also be achieved within a single membrane-enclosed space by means of the binding of key enzymes and metabolites. Thus, in the case of the cytoplasmic matrix, compartmentation by binding of soluble enzymes to the matrical structures may confer advantages over and above t...

SummaryHeating raw milk at 80 °C for 2·5–20 min was found to result in compositional changes in the milk fat globule membrane (MFGM). The yield of protein material increased with the duration of heating, owing to incorporation of skim milk proteins, predominantly β-lactoglobulin, into the membrane. Lipid components of the MFGM were also affected, with losses of triacylglycerols on heating.

The interactions between the omega-3 unsaturated fatty acids and peroxisomal function have been reviewed, in order to update and integrate knowledge in this area. Following a brief retrospective of the major clinical involvements of these fatty acids, the participation of the peroxisome in their metabolism has been appraised-the peroxisome being shown to exert a major influence on both the synthesis and degradation of the omega-3 fatty acids, with these effects flowing on to the widespread physiological implications of the derivative eicosanoids. Interactions between the omega-3 and omega-6 families of fatty acids have been discussed, as have the interdependent phenomena of peroxisome proliferation, membrane remodelling and cellular signalling. Amongst the signalling involvements covered were those of steroid hormone receptor superfamily, the phosphatidylcholine cycle, and the regulatory influences of oxygen free radicals. Comment has also been included on the separate biological roles of the individual omega-3 fatty acids, their influence on differential gene function, and on the molecular mechanisms of their pharmacological effects. It is concluded that the peroxisome is intimately involved in directing the metabolism and physiological influence of the omega-3 unsaturated fatty acids, and that this organelle merits much greater emphasis in future research aimed at unravelling the profound biological effects of these unique and multipotent compounds.

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A survey of the existing data on the interactions of glycolytic enzymes with the cellular structure in mammalian tissues has substantiated the occurrence of an extensive degree of such associations in all tissues and during all stages of development. Furthermore, a considerable specificity was evident between the individual multiple forms of the enzymes in relation to these associations. In reviewing these data, a model has been developed which proposes that the glycolytic sequence is best described as consisting of a number of segments in vivo, each segment formed by a cluster of isozymes, many of which can interact with the actin containing filaments of the cytomatrix. The novel features of this segmentation and compartmentation have been described, and evidence has been provided that these phenomena collectively play a key role in meeting the different types of energy requirement in the cytoplasm of divergent cell types, with the wide selection of isozymes in this system offering the potential for increased flexibility and control in this important area of metabolism.

A purified arylesterase preparation from bovine plasma was characterized to the extent that it has a partial specific volume of 0.91ml/g and an apparent z-average molecular weight of 440000. The relatively large magnitude of the former reflects the presence of phospholipids, cholesterol, triglycerides and beta-carotene, the last-named being responsible for the pronounced yellow colour of the preparation. Removal of the lipid material is accompanied by a decrease in the apparent z-average molecular weight to 120000, the size of the smallest species detected by high-speed sedimentation equilibrium being in the vicinity of 70000 daltons: denaturation of the lipid-free preparation with 6m-guanidine hydrochloride caused essentially complete breakdown into subunits of this size. In kinetic studies on the enzyme the maximal velocity for the hydrolysis of phenyl acetate was found to increase by 60% on addition of 1 mm-Ca(2+), with the K(m) showing a concomitant decrease from 6.6 to 2.1 mm. Removal of lipid had no detectable effect on V(max.) or K(m) in either the presence or the absence of Ca(2+). It is concluded that the bovine plasma arylesterase preparation is either a lipoprotein or an enzyme-lipoprotein complex with properties very similar to those of the alpha(1)-lipoprotein or high-density lipoprotein (HDL(2)) fraction of serum.