Liver Mitochondria

Ovalbumin Conalbumin Ovomucoid Lysozyme Vitellogenin apo-VLDL Glucose-6-P-dehydrogenase Oviduct Oviduct (liver) Oviduct Oviduct Liver Liver Uterus Thyroid Hormones Carbamyl phosphate synthase Growth hormone Prolactin ( ) a-Glycerophosphate dehydrogenase Malic enzyme Liver Pituitary Pituitary Liver (mitochondria) Liver... 

ALASl. This repression-derepression mechanism is depicted diagrammatically in Figure 32-9. Thus, the rate of synthesis of ALASl increases greatly in the absence of heme and is diminished in its ptesence. The turnover rate of ALASl in rat liver is normally rapid (half-life about 1 hour), a common feature of an enzyme catalyzing a rate-limiting reaction. Heme also affects translation of the enzyme and its transfer from the cytosol to the mitochondrion. 

The theory has been advanced that there exists in the cell a particulate structure somewhat smaller than the mitochondrion, the lyso-some, that contains certain autolytic enzymes in a latent situation. The lysosome theory55,56 is very largely based upon measurements made in sucrose homogenates of rodent liver. Although the results for a-D-mannosidase in this tissue (see Table IV) are not incompatible with the theory, the results for other tissues do not always conform to it. In particular, the contrast between mouse and rat spleen argues against a universal single particle to which a-D-mannosidase is confined. Apart from the results quoted in Table IV, not much work has been done on the intracellular location of a-D-mannosidase. 

Compart mentation in fi Oxidation Free palmitate is activated to its coenzyme A derivative (palmitoyl-CoA) in the cytosol before it can be oxidized in the mitochondrion. If palmitate and [ CJcoenzyme A are added to a liver homogenate, palmitoyl-CoA isolated from the cytosolic fraction is radioactive, but that isolated from the mitochondrial fraction is not. Explain. 

T12. Fatty Acid Oxidation in Uncontrolled Diabetes When the acetyl-CoA produced during /3 oxidation in the liver exceeds the capacity of the citric acid cycle, the excess acetyl-CoA forms ketone bodies—acetone, acetoacetate, and D-/3-hydroxybutyrate. This occurs in severe, uncontrolled diabetes because the tissues cannot use glucose, they oxidize large amounts of fatty acids instead. Although acetyl-CoA is not toxic, the mitochondrion must divert the acetyl-CoA to ketone bodies. What problem would arise if acetyl-CoA were not converted to ketone bodies How does the diversion to ketone bodies solve the problem ... 

FIGURE 19-1 Biochemical anatomy of a mitochondrion. The convolutions (cristae) of the inner membrane provide a very large surface area. The inner membrane of a single liver mitochondrion may have more than 10,000 sets of electron-transfer systems (respiratory chains) and ATP synthase molecules, distributed over the membrane surface. Heart mitochondria, which have more profuse cristae and thus a much larger area of inner membrane, contain more than three times as many sets of electron-transfer systems as liver mitochondria. The mitochondrial pool of coenzymes and intermediates is functionally separate from the cytosolic pool. The mitochondria of invertebrates, plants, and microbial eukaryotes are similar to those shown here, but with much variation in size, shape, and degree of convolution of the inner membrane. 

Mitochondria are present in all eukaryotic cells that use oxygen in respiration, but the number per cell and the form and size vary.1-4 Certain tiny trypanosomes have just one mitochondrion but some oocytes have as many as 3 x 105. Mammalian cells typically contain several hundred mitochondria and liver cells5 more than 1000. Mammalian sperm cells may contain 50-75 mitochondria,6 but in some organisms only one very large helical mitochondrion, formed by the fusion of many individual mitochondria, wraps around the base of the tail. Typical mitochondria appear to be about the size of cells of E. coli. However, study of ultrathin serial sections of a single yeast cell by electron microscopy has shown that, under some growth conditions, all of the mitochondria are interconnected.7... 

What are the molar concentrations of the electron carriers in mitochondrial membranes In one experiment, cytochrome b was found in rat liver mitochondria to the extent of 0.28 pmol/g of protein. If we take a total mitochondrion as about 22% protein, the average concentration of the cytochrome would be 0.06 mM. Since all the cytochromes are concentrated in the inner membranes, which may account for 10% or less of the volume of the mitochondrion, the concentration of cytochromes may approach 1 mM in these membranes. This is sufficient to ensure rapid reactions with substrates. 

Electron micrograph of a thin section of a liver mitochondrion. Courtesy of Dr. Daniel S. Friend... 

A similar shuttle, the malate-aspartate shuttle, operates in heart and liver (Fig. 6). Oxaloacetate in the cytosol is converted to malate by cytoplasmic malate dehydrogenase, reoxidizing NADH to NAD+ in the process. The malate enters the mitochondrion via a malate-a-ketoglutarate carrier in the inner mitochondrial membrane. In the matrix the malate is reoxidized to oxaloacetate by NAD+ to form NADH. Oxaloacetate does not easily cross the inner mitochondrial membrane and so is transaminated to form aspartate which then exits from the mitochondrion... 

Glutamate too may be produced from oxoglutarate And now that it s been introduced deamination is its fate For inside each mitochondrion of every liver cell Is GDH...(reducing NAD as well). 

The concentration of free pantothenic acid in the liver is about 15 xM that in the heart is about tenfold greater (Robishaw and Neely, 1985). The concentration of the cofactor form of the vitamin, coenzyme A, is higher in the mitochondrion than in the cytosol. In the Ever, cytosolic coenzyme A is about 0.06 mM, and mitochondrial coenzjmie A, about 2.6 mM. In the liver, about 70% of coenzyme A is mitochondrial, whereas in the heart about 95% is mitochondrial (Tahiliani and Neely, 1987). These values might be compared with that for carnitine, another molecule used in the handling of fatty acids. Please consult the Carnitine section in Chapter Four. About half of the coenzjrme A in liver occurs as the long-chain fatty acyl-coenzyme A derivative. The concentration of fatty acid s)mthase in the cytoplasm is quite low, about 0.01 pM. Hence, the concentration of the 4 -phospho-pantetheine cofactor is much lower than that of coenzyme A. The pantothenic acid boimd to this enzyme does not make a significant contribution to our dietary vitamin. 

D. Ketone bodies are synthesized in the liver from fatty acids derived from the blood. During the cytosolic activation of the fatty acid, ATP is converted to AMP. Carnitine is required to carry the fatty acyl group across the mitochondrial membrane. In the mitochondrion, the fatty acid is oxidized. Acetyl CoA and acetoacetyl CoA are produced and react to... 

B. This compound is acetoacetate, which is synthesized in the liver when blood insulin levels are low. HMG CoA synthetase is the key regulatory enzyme for synthesis, not oxidation. Acetoacetate is transported to tissues, such as muscle, where it is activated in the mitochondrion by succinyl CoA (not ATP), cleaved to 2 acetyl CoA, and oxidized via the TCA cycle, which requires the vitamin thiamine as thiamine pyrophosphate, a cofactor for a-ketoglutarate dehydrogenase. Biotin is not required. 

The complex tetrapyrrole ring structure of heme is built up in a stepwise fashion from the very simple precursors sue-cinyl-CoA and glycine (Figure 32-2). The pathway is present in all nucleated cells. From measurements of total bilirubin production, it has been estimated that daily synthesis of heme in humans is 5 to 8mmol/kg body weight. Of this, 70% to 80% occurs in the bone marrow and is used for hemoglobin synthesis. Approximately 15% is synthesized in the liver and is used to produce cytochrome P-450, mitochondrial cytochromes, and other hemoproteins. The pathway is compartmentalized, with some steps occurring in the mitochondrion and others in the cytoplasm. Little is known about the transport of intermediates across the mitochondrial membrane, and no transport defect has yet been reported in the porphyrias. 

In Vitro Studies. An assay for measuring taurine N-acyltransferase activity with C-phonylacetyl CoA as substrate has boon described (79). Enzyme activity was located In the matrix of the mitochondrion and was usually higher In kidney than liver for the nine fish species studied. Studies with Isolated renal tubules from the winter flounder and the southern flounder (84 and James and Pritchard, unpublished) have shown that added phenylacetlc acid and benzoic acid were extensively metabolized to their taurine conjugates during a 60 minute Incubation. 

Ketogenesis The production of ketone bodies by the liver in response to increased P-oxidation with a decreased rate of the Krebs cycle as a resnlt of shnttling acids from the mitochondrion for the synthesis of glncose via glnconeogenesis. 

Figure 4 Immunogold localization of mannose-terminal glucocerebrosidase in rat liver Kupffer cells 30 minutes after enzyme infusion. Gold particles are concentrated in the lysosomes. Lys = Lysosome M = mitochondrion Nuc = nucleus PM = plasma membrane. Original magnifications (A) x10,000 (B) x33,000. Bars = 1 pm. (From Ref. 24.)...

Although fatty acid synthesis occurs within the cytoplasm of most animal cells, liver is the major site for this process. (Recall, for example, that liver produces VLDL. See p. 349.) Fatty acids are synthesized when the diet is low in fat and/or high in carbohydrate or protein. Most fatty acids are synthesized from dietary glucose. As discussed, glucose is converted to pyruvate in the cytoplasm. After entering the mitochondrion, pyruvate is converted to acetyl-CoA, which condenses with oxaloacetate, a citric acid cycle intermediate, to form citrate. When mitochondrial citrate levels are sufficiently high (i.e., cellular energy requirements are low), citrate enters the cytoplasm, where it is cleaved to form acetyl-CoA and oxaloacetate. The net reaction for the synthesis of palmitic acid from acetyl-CoA is as follows ... 

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