Intracellular Compartmentation of Organelles and Gradients of Low Molecular Weight Species

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Intracellular compartmentation of metabolites without intervening membranes is an important concept that has emerged from consideration of the metabolic inhomogeneities associated with a highly organized and structured cytoplasm within mammalian cells. This recognition is primarily due to the development of experimental approaches to measure metabolite or ion concentrations at specific subcellular sites, thereby providing a means to study concentration gradients within the aqueous cytoplasm in intact cells. The presence of mitochondrial clusters has been shown to create gradients of low molecular weight species, such as O2, ATP, and pH, with important implications for substrate supply for function and regulation of cellular processes. Moreover, the existence of kinetically distinct precursor pools has been shown to result in functional compartmentation of biochemical pathways, such as DNA replication and carbohydrate metabolism. The creation of these specialized microzones of metabolism in accordance with their association with cellular organelles or membranal structures may be integral to normal function and regulation of adult mammalian cells.

Introduction

The concept that asymmetry occurs within mammalian cells has long been recognized. The intrinsic asymmetry within the cytoplasm of differentiated cells is reflected in the polarized distribution of cell ultrastructures and the associated heterogeneous chemistry and zonation of metabolism. The initial studies that elegantly demonstrated an association of asymmetry of structures with asymmetry of function within cells were described over 30 years ago by Garfinkel (1963) and Garfinkel and Lajtha (1963). They found that exogenously added radiolabeled glycine was preferentially utilized for hippurate biosynthesis in the mitochondria and did not equilibrate with the cellular glycine pool in kidney (Garfinkel, 1963, Garfinkel and Lajtha, 1963). This selective partitioning of glycine corresponded to the localization of high density of mitochondria adjacent to the plasma membrane of the cells. Thus, despite the absence of membranal barriers, metabolic inhomogeneity occurs within cell cytoplasm due to functional compartmentation of metabolites and substrates with cellular organelles.

In more recent years, our laboratory has revisited the concept of metabolic inhomogeneity and its significance for cell function. We found that the spatial distribution of mitochondria within liver and kidney cells can create metabolite gradients and intracellular compartmentation of high flux systems (Aw and Jones, 1985, Jones, 1986, Jones et al., 1987), thus resulting in specific microcompartmentation of metabolism within the cytoplasmic milieu of these cells. Depending on metabolite and substrate fluxes and consumption, as well as on diffusional distances, the intracellular compartmentation of low molecular weight species may be a crucial and generalized mechanism for optimizing specialized cell functions within mammalian cells. We have termed this metabolic inhomogeneity “microzonation,” to designate specific zones of heterogeneous metabolism within the cytoplasm. Conceptually, microzonation of function within cells is analogous to the zonation of metabolism in different regions of an organ, such as the liver, as described by Jungermann (1988).

Despite the obvious fundamental importance of intracellular metabolic heterogeneity in regulation of cell function, research progress in this field has been slow. It is noteworthy that the major concepts of intracellular compartmentation, diffusion-mediated metabolite gradients, inhomogeneity of structure and function, that were proposed years earlier are still viable today. This review summarizes our current knowledge on this subject. The chapter will focus on the specific concepts pertaining to intracellular compartmentation and gradients of low molecular weight species in association with heterogeneous distribution of cell organelles, such as the mitochondria. This discussion will be pertinent to understanding metabolic inhomogeneity within the cytoplasmic milieu of cells in the absence of delineating membranal barriers and the influence of such heterogeneous chemistry on control of cell function.

Section snippets

Compartmentation of Mitochondria within Mammalian Cells

Mitochondrial oxidative phosphorylation essentially provides the energy input that is needed for the highly specialized structures and functions of differentiated cells. The volume occupied by mitochondria in cells is highly variable and can range from 15 to 50% of the total cell volume. The distance between the site of ATP production in the mitochondria and the site of ATP utilization, such as at the plasma membrane, can impose a limitation to optimal ATP supply and consumption, thus

Intracellular Gradients of O2

The initial studies on O2 diffusion in tissues and tissue oxygenation were pioneered by Krogh (1919). These earlier studies provided indications for imposed O2 gradients, but the factors that affect intracellular O2 supply have not been rigorously defined.

Within cells, mitochondria are responsible for most of the O2 consumption. Isolated mitochondria have been studied extensively with regard to their O2 dependence as a function of O2 concentration. The results show that the P50 value for

Intracellular Gradients of Metabolites and Substrates

Because of the preferential partitioning of mitochondria to discrete regions of the cytoplasm of cells, a unique pattern of microzonation of ATP within the aqueous cytoplasm is created in accordance with its association with the mitochondria. The resultant microheterogeneities in metabolite and substrate distribution may be important in regulation of cell structure and function.

Determinants of Regional pH Differences

The kidney plays a pivotal role in organismic acid-base balance and, because of the well-defined polarized distribution of mitochondria to the basal region of the proximal tubule cell, it offers an ideal model for studying regional difference in pH within mammalian cells. In this cell, transcellular movement of acid and base equivalents is effected by the function of the pH-controlling systems that are localized to the opposite poles of the cell (Boron, 1986). At the brush border membrane, the

Concluding Remarks

Intracellular asymmetry is a unique characteristic of adult mammalian cells. At a structural and organellar level, cytoplasmic inhomogeneity is best reflected in heterogeneous distribution of mitochondria, which often appear in clusters or distinct zones. At a functional level, mitochondrial clustering causes steep gradients of low molecular weight species like O2, ATP, and pH, thereby resulting in metabolite inhomogeneities at different cytoplasmic sites without membranal delineation.

Acknowledgments

Research in the author’s laboratory was supported by grants from the National Institutes of Health GM-36538, GM-28176 (to D. P. Jones), and DK-4510, DK-43785 (to T. Y. Aw). T. Y. Aw is a recipient of an Established Investigatorship Award from the American Heart Association.

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