Regulation of heme synthesis in the regenerating rat liver

This investigation shows that the regulation of heme synthesis in the regenerating rat liver does not differ from the regulation in the normal liver. The heme saturation of tryptophan pyrrolase was found to be low, indicating a reduced concentration of heme in the regulatory heme pool of the regenerating rat liver. As expected, ALAS in the mitochondrial fraction was found to be elevated. It was also shown that ALAS in the regenerating rat liver can be induced by the porphyrinogenic drugs AIA and DDC and that heme reduces its activity. The decrease observed in the activity of cytosolic ALAS might be due to impaired synthesis of the enzyme but does not affect the regulation of the heme biosynthetic pathway.     

Regulation of phosphatidylcholine synthesis in rat liver endoplasmic reticulum.

The biosynthesis of phosphatidylcholine in rat liver microsomal preparations catalysed by CDP-choline-1,2-diacylglycerol cholinephosphotransferase (EC 2.7.8.2) was inhibited by a combination of ATP and CoA or ATP and pantetheine. ATP alone at high concentrations (20 mM) inhibits phosphatidylcholine formation to the extent of 70%. In the presence of 0.1 mM-CoA, ATP (2 mM) inhibits to the extent of 80% and in the presence of 1 mM-pantetheine to the extent of 90%. ADP and other nucleotide triphosphates in combination with either CoA or pantetheine are only 10-30% as effective in inhibiting phosphatidylcholine synthesis. AMP(CH2)PP [adenosine 5'-(alphabeta-methylene)triphosphate] together with CoA inhibits to the extent of 59% and with pantetheine by 48%. AMP-P(CH2)P [adenosine 5'-(betagamma-methylene)triphosphate] together with either CoA or pantetheine had no significant effect on phosphatidylcholine formation. Other closely related derivatives of pantothenic acid were without effect either alone or in the presence of ATP, as were thiol compounds such as cysteine, homocysteine, cysteamine, dithiothreitol and glutathione. Several mechanisms by which this inhibition might take place were ruled out and it is concluded that ATP together with either CoA or pantetheine interacts reversibly with phosphatidylcholine synthetase to cause temporarily the inhibition of phosphatidylcholine formation.     

Regulation of protein synthesis in intact rat liver by calcium mobilizing agents

1. Exposure of intact perfused rat liver to EGTA, vasopressin or phenylephrine resulted in a rapid decrease in polysome formation. Pretreatment with phentolamine, an alpha-adrenergic antagonist, blocked the effect of phenylephrine. 2. Hormonal inhibitions of leucine incorporation into protein in isolated hepatocytes and of polysome formation in perfused liver were reversed in the presence of supraphysiologic extracellular Ca2+ concentrations. 3. The beta-adrenergic agonist isoproterenol exerted minimal effects on polysome content. 4. It is proposed that intracellular Ca2+ stores sensitive to hormonal modulation are necessary for maintenance of protein synthesis in hepatocytes.     

Effect of post-ischaemic recovery on albumin synthesis and relative amount of translatable albumin messenger RNA in rat liver.

In liver cells recovering from reversible ischaemia, total protein synthesis by postmitochondrial supernatant and membrane-bound and free polyribosomes is not different from that in sham-operated controls. However, the relative proportion of specific proteins is changed, since the incorporation of [3H]leucine in vivo into liver albumin, relative to incorporation into total protein, as determined by precipitation of labelled albumin with the specific antibody, decreases by 40-50% in post-ischaemic livers. Cell-free synthesis by membrane-bound polyribosomes and poly(A)-enriched RNA isolated from unfractionated liver homogenate shows that the decrease in albumin synthesis in liver of rats recovering from ischaemia is due to the relative decrease in translatable albumin mRNA.     

The effects of amino acids on albumin synthesis by the isolated perfused rat liver

Albumin synthesis was measured in the isolated perfused rat liver by using the livers of both well-fed and starved rats. Starvation markedly decreased albumin synthesis. The livers from starved rats were unable to increase synthesis rates after the addition to the perfusates of single amino acids or the addition of both glucagon and tryptophan. Arginine, asparagine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, threonine, tryptophan and valine, added together to ten times their normal peripheral blood concentrations, restored synthesis rates to normal. The plasma aminogram (i.e. the relative concentrations, of amino acids) was altered by depriving rats of protein for 48h. The use of blood from the deprived rats as perfusate, instead of normal blood, decreased albumin synthesis rates significantly by livers obtained from well-fed rats. The addition of single amino acids, including the non-metabolizable amino acid, alpha-aminoisobutyric acid, to the above mixture increased albumin synthesis rates to normal values. It is concluded that amino acids play an important role in the control of albumin synthesis and that more than one mechanism is probably involved.     

Regulation of polyamine synthesis in chicken liver

• 1.1. Insulin injected into fasting chickens promotes a marked increase of liver S-adenosylmethionine decarboxylase, with maximum activity 6 hr after administration.
• 2.2. The apparent half-life of the enzyme is 22 min.
• 3.3. Ornithine and putrescine in vivo give rise only to small modifications in enzyme activity; spermidine and spermine induce the enzyme.
• 4.4. Putrescine activates the decarboxylase in vitro. Spermidine and spermine, however, appear to be inhibitors.
• 5.5. Insulin promotes a decrease of arginine, ornithine and all other amino acids, except for proline which increases. Putrescine is enhanced, spermidine slightly decreased and spermine unchanged.
• 6.6. Levels of polyamines are significantly altered by administration of methionine which promotes synthesis of spermidine, and by spermine which blocks transformation of putrescine into spermidine.

     

Estrogen dramatically decreases albumin mRNA levels and albumin synthesis in Xenopus laevis liver

The levels of albumin mRNA in Xenopus laevis liver were measured at various times after injection of estradiol using two different methods involving hybridization of cloned albumin cDNA to total liver RNA. The absolute levels of albumin mRNA fell by more than 95% during the first 4 days following estrogen treatment, then slowly returned to normal levels over the following 12 days. Albumin synthesis paralleled the albumin mRNA levels during the first 8 days after injection; but, 16 and 32 days after injection, albumin synthesis again decreased while albumin mRNA remained at normal levels. The time courses of the effects of estrogen on albumin and vitellogenin mRNA levels were different. Whereas albumin mRNA levels were minimal 4 days after estradiol injection, vitellogenin mRNA levels were maximal 8 days after injection.     

Cytological studies of plasma albumin in the rat liver

Summary The presence of albumin in the parenchymal cells of the rat liver is demonstrated using the fluorescent antibody method. It is evident that there is no immunological difference between liver and blood albumin. The liver section is not uniformly stained by the fluorescent antibody and this may be due to the uneven distribution of albumin in the liver. A small percentage of the parenchymal cells presumably containing albumin are markedly fluorescent. The nuclei of these cells are not fluorescent implying the absence of albumin and this is consistent with the fact that nuclei are not involved in synthesis of albumin. The scattered distribution of brightly fluorescent cells confirms earlier observations that all parenchymal cells are not functionally alike, although they appear homogenous histologically.     

Regulation of cholesterol synthesis in the liver and mammary gland of the lactating rat.

The activity of 3-hydroxy-3-methylglutaryl-CoA reductase (HMG-CoA reductase; EC 1.1.1.34) in the lactating mammary gland of rats killed between 10:00 and 14:30 h was 2-3 times that in the livers of the same animals. In contrast, after injection of 3H2O in vivo, the rate of appearance of 3H in the cholesterol of the gland was much lower than that in the liver. In the mammary gland of virgin and non-lactating animals, the activity of HMG-CoA reductase was less than 10% of that of the lactating gland. The activity of HMG-CoA reductase in the lactating mammary gland was significantly (P less than 0.005) lower at midnight than at mid-day, and appeared to show an inverse relationship to the activity of the liver enzyme. However, there was no corresponding change in the incorporation of 3H into the gland cholesterol. Withdrawal of food for 6h had no effect on the activity of HMG-CoA reductase in the lactating mammary gland, but resulted in a significant decrease (P less than 0.005) in that of the liver. Starvation of lactating rats for 24h produced a significant decrease (P less than 0.005) in the activity of the enzyme in both organs. There was also a significant decline in the rate at which 3H2O was incorporated in vivo into the cholesterol of both organs (liver, P less than 0.05; gland, P less than 0.005). Giving a high-fat palatable diet together with chow to lactating animals led to a decline in HMG-CoA reductase activity in the mammary gland, but not in liver. This decrease in the gland was not accompanied by a corresponding decline in the apparent rate of cholesterol synthesis.