Compartmentation of metabolites

Leegood, R. C. 1985. The inter-cellular compartmentation of metabolites in leaves of Zea mays. Planta. 164,163-171. 

Unfortunately, practically nothing is known about the compartmentation of CAM metabolites other than the organic acids. Of course, starch can easily be recognized as a constituent of chloroplasts (see Fig. 2.8). Further investigations to clarify compartmentation of metabolites are desirable to solve the problem of cellular control of CAM. 

J. Guern, J. P. Renaudin, S. C. Brown, The compartmentation of secondary metabolites in plant cell cultures. Cell Culture and Somatic Cell Genetics of Plants. Vol. 4 (F, Constabel and 1. K. Vasil, eds.). Academic Press, San Diego, 1987, p. 43. A. L. Samuels, M. Fernando, and A. D. M. Glass, Immunofluorescent localization of plasma membrane H -ATPase in barley roots and effects of K nutrition. Plant Physiol. 99 1509 (1992). 

Berl, S., Nicklas, W. J. and Clarke, D. D. Compartmentation of citric acid cycle metabolism in brain labelling of glutamate, glutamine, aspartate and gaba by several radioactive tracer metabolites. J. Neurochem. 17 1009-1015, 1970. 

Wink M (1997) Compartmentation of secondary metabolites and xenobiotics in plant vacuoles. In Leigh RA, Sanders D, Callow JA (eds) The plant vacuole advances in botanical research, vol 25. Academic, London, pp 141-169... 

The structure of a compartmental PK model is given by the number of compartments being used and the way the compartments are connected. For most drugs the plasma concentration-time profiles can be sufficiently described by one-, two-or three-compartment models. A one-compartment model assumes that no time is necessary for the distribution of the drug and the whole distribution occurs within this one compartment. The two-compartment model implements in addition to a central compartment a peripheral compartment which allows the description of distribution processes of the compound in, e.g. tissues with different physicochemical properties. The three-compartment model provides an additional compartment for distribution processes. The use of more than three compartments is quite rare, but there are some situations where the model can get quite difficult, e.g. if the concentration-time profile of metabolites are also considered within the model. For more information regarding compartment models, refer to Rowland and Tozer... 

Some, but not all, of the benefits conferred by compartmentalization inside lipid bilayer membranes can be achieved by surfaces. Surfaces allow concentration of metabolites, and, if the surface is a mineral that is not in redox equilibrium with its surroundings, a surface can provide a source of energy. Compartmentalization can also be achieved inside porous minerals. For example, the walls of hydrothermal vents are porous and trap organic material6 and may indeed have provided the first compartmentalization that allowed the emergence of metabolism and macromolecules in a protected environment.7 Also, there are tiny pores in rocks, including tubes in chroysotile, and there are microcracks in quartz, both of which could support diffusion and dispersion by currents. 

The rate of photosynthesis does not depend on the amount of a single component (e.g., the activity of a particular enzyme). There is a wide range of possible regulatory factors, proven to exist in vitro, but the importance of which in vivo has still to be determined. In particular, there is a multitude of factors affecting the activity of the enzymes involved, with pH, ions, coenzymes, and metabolite effectors modulating the activity of every enzyme studied thus far. Compartmentation is the other key factor. The role of metabolite transport in the cell, particularly between chloroplast and cytosol, but also to and from mitochondria, vacuole, and other organelles, is now considered to be fundamental to the regulation of photosynthesis. In this chapter, we look at the factors considered to be of major importance... 

Guern, J., Renaudin, J.P and Brown, S.C. (1987) The compartmentation of secondary metabolites in plant cell cultures, in Cell Culture and Somatic Cell Genetics (eds R Constabel and I. Vasil). Academic Press, New York, pp. 43-76. 

Plate 3 Compartmentation of biosynthesis and sequestration. Abbreviations SM, secondary metabolites GS-SM, conjugate of SM with glutathione NPAAs, non-protein amino acids ATP, adenosine triphosphate ADP, adenosine diphosphate mt, mitochondrium cp, chloroplast nc, nucleus 1, passive transport 2, free diffusion 3, H+/SM antiporter 4, ABC transporter for SM conjugated with glutathione 5, ABC transporter for free SM 6, H+-ATPase. (Fig. 1.4, p. 9)... 

During the light period, when COi is being fixed in the chloroplasts by the RPP pathway, it is likely that the mechanisms discussed above for C3 photosynthesis are also functional in CAM plants [19]. Additional control mechanisms are expected to provide for the efficient functioning of the diurnal cycle of CO2 fixation. The functioning of CAM cannot, however, be interpreted solely in terms of enzymolo-gy, but rather will involve cellular compartmentation of enzymes and metabolites together with intracellular transport processes [17]. 

Steroid hormones by the application of isotope tracer techniques 3I8-321]. Lieberman and co-workers undertook urinary metabolic studies [304,325] by administration of isotopically labeled steroids followed by the determination of specific activities of urinary metabolites derived only from the administered substance. Mathematical analyses were performed by compartmentalization of the metabolism [385]. One- or two-compartment systems are most frequently used [304,306,384,386,387]. A compartment is defined as a particular species (e.g., dehydroepiandro-sterone sulfate) in a particular space (e.g., peripheral organ). It is assumed that a species entering a compartment mixes immediately with the whole compartment. Two factors were responsible for the institution of com-partmental analysis ... 

It has long been recognised that metabolites, and metabolic activities, are distributed non-uniformly within cells. Such compartmentation is undoubtedly an important aspect of metabolic regulation. The accurate quantitation of metabolite levels in various intracellular compartments represents a major stumbling block in the study of metabolic regulation. There are two aspects to the study of metabolite compartmentation by in vivo NMR. The first, and most problematic, is the assignment of resonances to specific intracellular compartments we focus on this aspect here. The second is the use of NMR spectroscopic parameters (e.g., intensity, chemical shift) to monitor conditions (e.g., pH, concentrations, fluxes) within specific compartments, using methods outlined in other sections of this chapter. 

Coenzyme A serves several purposes. It is a high-energy compound, activating the initial steps of the metabolic pathway. It is used as a tag to earmark a molecule for a particular pathway. It is large and cannot cross membranes, so compartmentalization of pathways can be affected by binding metabolites to coenzyme A. 

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