Thymidine kinase and deoxyribonucleic acid metabolism in growing and regenerating livers from rats on controlled feeding schedules

Somatostatin therapy protects porcine livers in small-for-size liver transplantation

Small-for-size (SFS) injury occurs in partial liver transplantation due to several factors, including excessive portal inflow and insufficient intragraft responses. We aim to determine the role somatostatin plays in reducing portal hyperperfusion and preventing the cascade of deleterious events produced in small grafts. A porcine model of 20% liver transplantation is performed. Perioperatively treated recipients receive somatostatin and untreated controls standard intravenous fluids. Recipients are followed for up to 5 days. In vitro studies are also performed to determine direct protective effects of somatostatin on hepatic stellate cells (HSC) and sinusoidal endothelial cells (SEC). At reperfusion, portal vein flow (PVF) per gram of tissue increased fourfold in untreated animals versus approximately threefold among treated recipients (p = 0.033). Postoperatively, markers of hepatocellular, SEC and HSC injury were improved among treated animals. Hepatic regeneration occurred in a slower but more orderly fashion among treated grafts; functional recovery was also significantly better. In vitro studies revealed that somatostatin directly reduces HSC activation, though no direct effect on SEC was found. In SFS transplantation, somatostatin reduces PVF and protects SEC in the critical postreperfusion period. Somatostatin also exerts a direct cytoprotective effect on HSC, independent of changes in PVF.

The role of thymidine kinase in cancer diseases

Thymidine kinase 1 (TK 1-fetal) is a cell cycle-dependent marker that increases dramatically during the S-phase of the cell cycle. In this review, the authors discuss serum levels of thymidine kinase in a variety of neoplasias. Determination of thymidine kinase helps to monitor the follow-up of solid tumours and haematological malignancies as well as indicating the efficacy of adjuvant and palliative chemotherapy. Elevated levels of thymidine kinase must always be interpreted together with a detailed knowledge of the patient's condition because nonspecific elevations of serum levels (inflammatory and autoimmune diseases) must be excluded.

Decompression of the portal bed and twice-baseline portal inflow are necessary for the functional recovery of a "small-for-size" graft

BACKGROUND

In partial liver transplant, a reduction in the intrahepatic vascular bed produces a rise in the portal vein flow and the portal venous pressure gradient, leading to endothelial and, thereby, hepatocellular injury and death in a process known as "small-for-size" (SFS) syndrome.

OBJECTIVE

To demonstrate that a calibrated portocaval shunt prevents superfluous inflow in a porcine model of SFS transplant.

METHODS

Donor pigs (15-20 kg) underwent 70% hepatectomy. In 2 groups, a 6 mm (S6) (n = 6) or 12 mm (S12) (n = 6) Gore-Tex shunt was placed between the portal vein and infrahepatic inferior vena cava. In a third group, no portocaval shunt was placed (SFS) (n = 17). Grafts were stored for 5 hours at 4°C and then transplanted into recipients (30-35 kg).

RESULTS

Five-day survival was 29% in SFS, 100% in S6, and 0 in S12. Postreperfusion portal vein flow was 4-, 2-, and 1-times flow at baseline in SFS, S6, and S12, respectively. With respect to portal venous pressure gradient, both the 6- and 12-mm shunts effectively decompressed the portal bed. Aspartate aminotransferase and bilirubin rose and the Quick prothrombin time fell in all animals after reperfusion but improved significantly by day 5 in S6. Serum levels of endothelin-1 remained elevated in SFS and S12 but returned to baseline by 12 hours in S6: 2.76 (2.05-4.08) and 2.04 (1.97-2.12) versus 0.43 (0.26-0.50) pg/mL, respectively (P < 0.05 for both comparisons).

CONCLUSIONS

A calibrated portocaval shunt that maintains portal vein flow about twice its baseline value produces a favorable outcome after SFS liver transplantation, avoiding endothelial injury due to portal hyperperfusion or to hypoperfusion because of excess shunting.

Portal hyperperfusion: mechanism of injury and stimulus for regeneration in porcine small-for-size transplantation

Understanding the pathogenesis of small-for-size (SFS) syndrome is critical to expanding the applicability of partial liver transplantation. We aimed to characterize its acute presentation and association with alterations in hepatic hemodynamics, microstructure, and regeneration in a porcine model. Eighteen SFS liver transplants were performed. Donors underwent 70% hepatectomy. Partial grafts were implanted into larger recipients. Whole liver transplants were also performed (n = 6). Recipients were followed until death or for 5 days. Hemodynamics were measured, and tissue was sampled intraoperatively and at the study end. Serum was sampled regularly during follow-up. Seventeen SFS transplants and 6 whole liver transplants were included. SFS grafts represented 23.2% (19.3%-25.3%) of the recipients' standard liver volume. The survival rate was 29% and 100% in the SFS and whole liver groups, respectively. The portal venous flow, pressure gradient, and resistance were significantly higher in recipients of SFS grafts versus whole livers after portal and arterial reperfusion. Arterial flow as a percentage of the total liver blood flow was significantly lower after reperfusion in SFS grafts and remained so when measured again after 5 days. Markers of endothelial cell injury increased soon after reperfusion, and those of hepatocellular injury increased later; both predicted the appearance of either graft failure or histological recovery. Proliferative activity peaked earlier and higher among nonsurvivors in the SFS group. Surviving grafts demonstrated a slower but maintained rise in regenerative activity, although metabolic activity failed to improve. In SFS transplantation in the acute setting, portal hyperperfusion is a stimulus for regeneration but may simultaneously cause irreparable endothelial injury. This porcine model not only helps to elucidate the inciting factors in SFS pathogenesis but also offers a clinically relevant means to study its prevention.

Liver regeneration: methods for monitoring and their applications

Liver regeneration is an essential component of the reparative process following liver injury and surgical resection. It can be assessed by different tissue-based tests such as liver weights, mitotic counts, DNA contents and synthesis rates, immunohistochemical staining of nuclear antigens, gene expressions and certain protein levels or various serum-based tests that largely consist of specific enzyme determinations or documentation of certain proliferation markers. Although the simplest tissue-based test of liver regeneration is measurement of liver weights, these determinations are influenced by the extent of deposition of various materials not directly related to regeneration, such as lipids, glycogen and blood volumes. Because mitosis constitutes a very short segment of the cell cycle, mitotic counts are infrequently observed by light microscopy. Thymidine and BrdU incorporation into DNA are the reference tools for studying DNA synthesis, but their use requires pre-injection with radioactive isotopes or nucleotides which render them impractical for human studies. Flow cytometry is an accurate and objective method of monitoring hepatic regenerative activity but requires sophisticated equipment that is not generally available in many laboratories. Immunohistochemical staining for nuclear antigens (Ki-67, proliferating cell nuclear antigen [PCNA], DNA polymerase alpha and nucleolar organizer region [NOR] proteins) are acceptable and commonly used methods of monitoring regenerative activity but are subject to inter- and intra-observer variability. Gene expression rates such as Histone-3 mRNA abundance are hampered by the relatively low rates of gene transcription and the need for recombinant DNA technology. Protein and enzyme levels in liver tissues, such as putrescine, ornithine decarboxylase and thymidine kinase, are not precise and are confounded by the nutritional status of the host. While PCNA protein levels measured by immunoblot hold promise as a simple, accurate and reproducible marker of liver regeneration, additional studies are required to determine if this is a valid marker of regenerative activity in various models of hepatic injury and in humans. Of the serum-based determinations: thymidine kinase, ornithine decarboxylase, fibronectin, alpha fetoprotein, and early pregnancy factor offer practical and non-invasive tools to monitor liver regeneration, but the sensitivity and specificity of these tests have yet to be determined. In conclusion, many tissue and serum-based methods have been employed in clinical and experimental studies to assess liver regeneration; however, a gold standard has yet to be identified. Because of the disadvantages inherent in each method, and until a new, more accurate marker is identified, clinicians and scientists should incorporate a minimum of two independent markers in studies of liver regeneration.

Relationship between plasma and hepatic cytosolic levels of ornithine decarboxylase (ODC) and thymidine kinase (TK) in 70% hepatectomized rats

Ornithine decarboxylase (ODC) and thymidine kinase (TK) are enzymes important for DNA synthesis, a process that is critical for cell renewal and regeneration. As such, they already have been used as surrogate markers of regeneration in tissue. In the present study, the activity of these two enzymes in plasma of rats and regenerating hepatic tissue following a 70% hepatectomy were determined. The results demonstrate that the changes in these enzyme activities in plasma reflect the changes obtained in the liver tissue. Thus, blood levels of ODC and TK can be used as a less invasive and nondestructive means of monitoring the regenerative response of the liver and possibly other tissues.

Thymidine kinase activity of the chick pineal gland

Pineal thymidine kinase activity of 1-week-old chicks in situ varied significantly throughout the day. However, the circadian rhythm of thymidine incorporation seen with cultured chick pineal glands was not accompanied by variations in level of thymidine kinase activity in vitro. Thus the circadian rhythm in rate of cumulative incorporation of thymidine by cultured chick pineal glands is not determined by a rhythm in rate of the first reaction of the complex series of reactions by which thymidine is incorporated into DNA.

Studies on the hyperplasia ('regeneration') of the rat liver following partial hepatectomy. Changes in lipid peroxidation and general biochemical aspects

Using the experimental model of partial hepatectomy in the rat, we have examined the relationship between cell division and lipid peroxidation activity. In rats entrained to a regime of 12 h light/12 h dark and with a fixed 8 h feeding period in the dark phase, partial hepatectomy is followed by a rapid regeneration of liver mass with cycles of synchronized cell division at 24 h intervals. The latter phenomenon is indicated in this study by pulses of thymidine kinase activity having maxima at 24 h, 48 h and 72 h after partial hepatectomy. Microsomes prepared from regenerating livers show changes in lipid peroxidation activity (induced by NADPH/ADP/iron or by ascorbate/iron), which is significantly decreased relative to that in microsomes from sham-operated controls, again at 24 h, 48 h and 72 h after the operation. This phenomenon has been investigated with regard to possible underlying changes in the content of microsomal fatty acids, the microsomal enzymes NADPH:cytochrome c reductase and cytochrome P-450, and the physiological microsomal antioxidant alpha-tocopherol. The cycles of decreased lipid peroxidation activity are apparently due, at least in part, to changes in microsomal alpha-tocopherol content that are closely associated in time with thymidine kinase activity.

Expression of c-raf-1 and A-raf-1 during regeneration of rat liver following surgical partial hepatectomy

The major objective of this study was to investigate the expression of members of the raf family of proto-oncogenes during rat liver regeneration. The steady-state level of expression of both c-raf-1 and A-raf-1 increased three- to fivefold 18-24 h following partial hepatectomy, and it returned to basal levels by 72 h. Expression of c-myc and Ha-ras mRNA was increased at 3 and 18-24 h, respectively, confirming previous reports. Increased steady-state levels of c-raf-1, A-raf-1, and Ha-ras mRNA were also detected in hepatocytes isolated from rat liver 24 h after partial hepatectomy. Thus, elevated expression of the raf genes closely correlated with that of Ha-ras, beginning at 12 h and reaching maximal levels during the first peak of DNA synthesis following partial hepatectomy.

Effect of an antiandrogenic H2 receptor antagonist on hepatic regeneration in rats

Because biochemical "feminization" of the liver in males is observed with hepatic regeneration and because the hepatic regenerative response in females is greater than that in males, the possibility that antiandrogens might potentiate liver regeneration was investigated. Before 70% hepatectomy, adult male Wistar rats were treated with cimetidine, an antiandrogenic H2 antagonist, at doses up to 10 times greater than those used clinically. Control animals received either the saline vehicle or ranitidine, an H2 antagonist without antiandrogenic properties. Treatment with cimetidine reduced the hepatic cytosolic androgen receptor content compared with ranitidine treatment. Hepatectomy caused a further reduction in androgen receptor activity in all groups. Hepatic cytosolic estrogen receptor activity was comparable in all groups throughout the study. Moreover, the rate of liver growth and the levels of ornithine decarboxylase and thymidine kinase activity induced as part of the regenerative response were similar in all groups. Thus, cimetidine, despite its ability to bind to androgen receptors, and ranitidine, an H2 receptor antagonist without antiandrogen action, do not modulate the hepatic regenerative response to a 70% partial hepatectomy.

Identification of biliprotein as a liver growth factor

We determined the concentration of biliprotein in plasma of rats at different times after partial hepatectomy. From the same plasma samples, we purified a liver growth factor previously characterized by our group. When we plotted the 14 points studied, a linear relationship was obtained (r = 0.999; p less than 0.001). This result, in addition to our group's recent identification of this liver growth factor as an albumin-bilirubin complex, strongly suggests that biliprotein is a liver growth factor.

Lipid peroxidation in regenerating rat liver

Rats entrained to a strictly regulated lighting and feeding schedule have been subjected to partial hepatectomy or a sham operation. In the partially hepatectomised animals the period of liver regeneration is characterised by regular bursts of thymidine kinase activity. Liver microsomes from rats, at times corresponding to maximum thymidine kinase activity, have much reduced rates of lipid peroxidation compared to control preparations: this is due in part to increased levels of lipid-soluble antioxidant at times of maximal DNA synthesis. This temporal relationship between thymidine kinase and lipid peroxidation is consistent with the view that lipid peroxidation is decreased prior to cell division.

Hepatic regenerative enzyme activity after pericentral and periportal lobular toxic injury

Pericentral and periportal liver injuries involving less than 50% of the parenchyma were produced with acetaminophen and allyl alcohol, respectively. Doses were selected to produce comparable peak serum malate dehydrogenase, sorbitol dehydrogenase, and SGPT activities. The regenerative response was assessed by serial measurements of hepatic thymidine kinase (TK) activity and ornithine decarboxylase (ODC) activity. The initial responses reflected in ODC activity were more or less similar. However, the ultimate regenerative response reflected by TK activity was almost three times as great after periportal injury as after pericentral injury, after allowing for differences in the extent of necrosis. Histologic examination also showed greater mitotic and tissue reparative responses after periportal injury. These results suggest that the concept of hepatocellular heterogeneity applies to the regenerative response of liver cells as well as the metabolic functions previously identified.

Hepatic regenerative stimulator substance in the rabbit. Relation to liver regeneration after partial hepatectomy

The occurrence and activity of hepatic regenerative stimulator substance was investigated in the partially hepatectomized rabbit and related to biochemical and morphological parameters of liver regeneration. Male rabbits were 60% hepatectomized by excising the Spigelian, left lateral and left central lobes of the liver leaving the gallbladder in situ. [3H]Thymidine incorporation into DNA, the fraction of labelled hepatocyte nuclei, the fraction of mitoses and thymidine kinase activity rose from basal levels at 30-40 h after hepatectomy and increased up to 12-fold at 40-60 h. After 7 days, proliferation parameters returned to near prae-hepatectomy values and 82% of the initial liver mass was restored. Hepatic regenerative stimulator substance was biologically active when prepared from rabbit livers between 18-30 h after partial hepatectomy. At 12 and 30 h after intraperitoneal injection of the extract into normal rats, hepatic DNA synthesis was stimulated up to 2-fold in a dose-dependent fashion. The biological activity was protease-sensitive and thus depended on a protein component of the extract. The data demonstrate the existence of hepatic regenerative stimulator substance in regenerating rabbit liver and suggest that it is implicated in the regulation of liver growth after partial hepatectomy.

Asymmetric distribution of exogenous thymidine among pyrimidine isostichs suggests compartmentalization of replicative DNA synthesis during regeneration in rat liver

The distribution of radioactivity among pyrimidine isostichs (or isoplyths) of DNA from 24-h regenerating rat liver was studied with [3H]Thd, [14C]orotate or with inorganic 32Pi. Expression of incorporated radioactivity as log10% of total radioactivity recovered for each of the 11 pyrimidine isostichs detected showed that radioactivity from [3H]Thd was asymmetrically distributed among the isostichs, i.e., 3H radioactivity failed to access regions of DNA yielding lower molecular weight pyrimidine isostichs as efficiently as it accessed regions yielding higher molecular weight pyrimidine isostichs. The thymine (T) content of isostichs exceeded that of cytosine (C), i.e., T/C ratios for the first 10 isostichs averaged 1.43 +/- 0.08 and 1.28 +/- 0.05, depending on the method of analysis; furthermore, the T/C ratio for isostich 1 was significantly higher than ratios for isostichs 2 through 10. Asymmetric distributions of [3H]Thd radioactivity also were seen at 18 or 30 h post-partial hepatectomy. Thus, radioactivity from [3H]Thd, a DNA precursor from the salvage pathway, failed to efficiently access lower molecular weight isostichs despite thymine enrichment, suggesting that thymine moieties were supplied from additional sources. Radioactivity from [14C]orotate accessed lower molecular weight pyrimidine tracts more efficiently than [3H]Thd, but less efficiently than it accessed higher molecular weight isostichs, resulting in an asymmetric distribution of 14C radioactivity. This result suggested that appreciable quantities of thymine and cytosine moieties utilized for DNA synthesis were supplied de novo, but other sources also were utilized. Radioactivity from 32Pi, a de novo precursor, was distributed symmetrically, i.e., the slope among lower molecular weight isostichs increased enough that it was indistinguishable from slopes for intermediate and higher molecular weight isostichs. Since 32P radioactivity among lower molecular weight isostichs reflects appreciable contributions of de novo phosphate moieties from both pyrimidine- and purine-containing deoxynucleoside triphosphates, opportunities for observing contributions of 32P radioactivity from pathways other than the de novo pathways appeared to lie beyond limits of detectability. The distribution of radioactivity from labeled DNA precursors among lower molecular weight pyrimidine tracts (a) indicate that thymine moieties are contributed by both salvage and de novo pathways; (b) support the possibility that cytosine moieties also are contributed by both pathways; and (c) support the 'replitase' concept for channeling dNTPs to replicating forks.

Nucleoside incorporation into DNA and RNA in acute leukaemia: differences between the various leukaemia sub-types

The incorporation of the labelled deoxyribonucleosides 3H-deoxythymidine (3H-TdR), H3-deoxycytidine (3H-CdR), 3H-deoxyadenosine (3H-AdR), 3H-deoxyguanosine (3G-GdR), 3H-deoxyuridine (3H-UdR) and of labelled uridine (3H-UR) into DNA and RNA was studied in bone marrow (BM) and peripheral blood (PB) cells from 10 normal donors and 11 patients with acute myeloblastic leukaemia, 13 with acute non-T non-B common ALL (c-ALL) and seven with thymic acute lymphoblastic leukaemia. 3H-TdR incorporation was highest into the DNA of normal BM cells, 3H-CdR into DNA in Thy-All and 3H-UdR into DNA of c-ALL cells. Purine deoxynucleoside (3H-AdR and 3H-GdR) incorporation was highest in AML cells and they were incorporated mainly into RNA indicating that before utilization they are partially degraded from the deoxyribose to the corresponding ribose form. In all but three leukaemia samples, the 3H-UdR/3H-TdR incorporation ratio was above the range found in normal bone marrow, suggesting the leukaemic cells are more dependent than a normal mixed marrow cell population on the de novo pathway of thymidylate synthesis. The incorporation of nucleosides by peripheral blood cells was usually much lower than by the corresponding bone marrow cells, irrespective of blast percentage.

Effects of dietary butylated hydroxytoluene and phenobarbital on the activities of ornithine decarboxylase and thymidine kinase in rat liver and lung

Rats were entrained to a 12 h dark/12 h light cycle with food (60% protein +/- 0.5% butylated hydroxytoluene or 0.05% phenobarbital) available only during the first 2 h of the dark period. Under these conditions liver ornithine decarboxylase (ODC) activity in control animals displayed a characteristic diurnal oscillation. In livers of rats fed butylated hydroxytoluene or phenobarbital ODC activity was not increased whereas thymidine kinase (TK) activity was stimulated 4--10 fold at 3 days. In lungs from the same animals ODC and TK activities were unchanged. In rats fed butylated hydroxytoluene for 3 days [3H]thymidine incorporation into DNA was increased in liver but decreased in lung.

The diverse effects of 5'-bromodeoxyuridine on enzyme activities in cultured H35 hepatoma cells

Reuber (H35) hepatoma cells were grown in medium containing 10(-5)M bromodeoxyuridine (BrdU), which was incorporated into their DNA. Cell growth rate was unaffected by BrdU for the first two generations, after which it was reduced by about 50%. The effect of BrdU incorporation on the activities of several enzymes with rapid turnover rates was examined to test the hypothesis that the synthesis of such enzymes will be preferentially inhibited by BrdU. Tyrosine amino-transferase (TAT) activity decreased by 70% within two generations whereas thymidine kinase activity remained at control values. PEP carboxykinase activity was unchanged during the first generation in BrdU-containing medium but, during the second, its activity increased by at least 30%. Ornithine decarboxylase levels decreased by about 50% only after two generations in the presence of BrdU. There appeared to be no simple relationship between turnover rates and the effect of BrdU on enzyme activity. Incorporation of BrdU was found to inhibit the induction of both TAT and PEP carboxykinase by dexamethasone and to enhance the inhibition of cell growth by this steroid. These results are discussed with respect to possible mechanisms of gene expression and development in both normal and neoplastic cells.

Thymidine kinase in megaloblastic anaemia

Thymidine kinase has been measured in phytohaemagglutinin (PHA)-stimulated lymphocytes from 13 normal subjects and eight patients with megaloblastic anaemia. The levels in normal subjects ranged from 0.20 to 2.10 units/mg protein (mean 0.903 units/mg protein) and in megaloblastic anaemia from 2.99 to 9.97 units/mg protein). All the patients showed raised levels of the enzyme which were partly but not completely reduced to normal by addition of folic acid in vitro. Vitamin B12 in vitro had a lowering effect in the five vitamin-B12-deficient patients and two patients with combined deficiencies but not in one 'pure' folate-deficient patient. Thymidine kinase activity was highest in the cells of the least anaemic patients, suggesting that the degree of anaemia in megaloblastic anaemia may be determined in part by the ability of the cells to utilize thymidine by the 'salvage' pathway when the de novo pathway of thymidylate synthesis is failing. The rise in thymidine kinase activity in megaloblastic anaemia is presumably due to induction of the enzyme. Addition of methotrexate or 5-fluorouracil, drugs known to inhibit de novo thymidylate synthesis, caused an increase in thymidine kinase activity in normal PHA-stimulated lymphocytes after 24 h (but not after 1 h) which could be completely blocked by addition of puromycin. Thymidine mono- and di-phosphate kinases were also measured in normal PHA-stimulated lymphocytes. The activities were substantially higher than that of thymidine kinase and their activities were unaffected by methotrexate addition.

Stimulated DNA synthesis in livers and kidneys induced to proliferate associated with unchanged thymidine and thymidylate kinase activities

Enhanced mitotic activity and stimulated DNA synthesis associated with unchanged activity of thymidine and thymidylate kinases were observed in mouse kidneys induced to proliferate by intracardiac injection of lead acetate, and in rat livers following repeated administration of 5-azacytidine. On the other hand, the enhanced thymidine kinase activity evoked by L-tryptophan given by intubation at later stages of liver regeneration was paralleled by the enhanced incorporation of thymidine into DNA only to a very small degree.

Folate metabolism in the rat liver during regeneration after partial hepatectomy

1. Folate metabolism was studied during the early phases of liver regeneration after partial hepatectomy in rats accustomed to eating during the first 8h of a daily 12h dark period. 2. The content of 5-CH(3)-H(4)folate was drastically decreased during the first hours of regeneration. 3. The total HCO-H(4)folate coenzymes showed a constant decrease during the first 3 days of regeneration, and a continuous interconversion between 5-HCO-H(4)folate and 10-HCO-H(4)folate. 4. 10-HCO-H(4)folate synthetase, serine hydroxymethyl-transferase and 5,10-CH(2)-H(4)folate dehydrogenase activities were relatively low during the first hours after the operation, and increased only several hours later. 5. The increase in enzyme activities showed a stepwise pattern, apparently due to an interaction between the regeneration process and the controlled feeding schedules.

Modification of the template capacity of liver chromatin for form-B ribonucleic acid polymerase by food intake in rats under controlled feeding schedules

Nuclei from liver of rats accustomed to eating during the first 8h of a daily 12h dark period demonstrate an increased capacity to synthesize RNA 6H after the beginning of the feeding period. 2. This increase is accompanied by a higher yield of extractable form-B DNA-dependent RNA polymerase activity. 3. The endogenous RNA polymerase activity associated with nuclear chromatin is also stimulated by food intake. Both purified and chromatin-associated form-B enzyme activities exhibit different ionic strength requirements after food intake. 4. The sensitivity of exogenous (added) form-B-enzyme to changes in ionic strength changes after feeding when chromatin is used as template. 5. Chromatin extracted from the liver of fed rats is a better template for form-B-enzyme than chromatin extracted from starved rats.

Thymidine kinase activity in cardiac muscle during embryomic and postnatal development

Cytoplasmic thymidine kinase from cardiac muscle of the rat has been characterized. It has a pH optimum of 9.0 and a K(m) value for thymidine of 1.6mum. The sedimentation coefficient of this enzyme in sucrose gradients is 4.5S, which represents a molecular weight of approx. 69000. Thymidine kinase prepared from cardiac muscle of foetal, neonatal and adult rats is inhibited by dTTP and dTDP; there is neither inhibition nor stimulation by dTMP, dCTP, dATP, dGTP or cyclic AMP. The activity of thymidine kinase in differentiating cardiac muscle of foetal and neonatal rats declines progressively with development, reaching adult values of almost zero by the fifteenth to seventeenth day of postnatal development. This represents a 70-fold decrease in enzyme activity from 3 days before birth to 17 days after birth. The loss of thymidine kinase activity in differentiating cardiac muscle correlates temporally with the cessation of DNA biosynthesis and the loss of cytoplasmic DNA polymerase activity in this tissue.

Diurnal rhythms of hepatic carbohydrate metabolism during development of the rat

The ;8+16' feeding schedule (8h feeding and 16h without food in each 24h cycle) was applied to nursing mother rats to study enzyme development in neonatal rats in the absence of solid food. A ;16+8' suckling schedule (16h with the mother and 8h while the mother is fed in a separate cage) was used to show that the increases in pyruvate kinase, glucokinase and aldolase B activities that occur in the late suckling period of liver development do not require the intake of solid food at this time. Their activities may, however, be modulated by the composition of the diet at the time of weaning. Adaptation to the composition of the diet can occur within one feeding period, and to the periodicity of food provision in one or two feeding periods. In the early neonatal period, diurnal rhythms of tyrosine aminotransferase, liver glycogen and glucokinase are either greatly suppressed or absent, but develop rapidly after weaning. Food-dependent rhythms of glycogen and tyrosine aminotransferase were included in the late suckling period (day 14).

Isoenzyme patterns in parenchymal and non-parenchymal cells isolated from regenerating and regenerated rat liver

Parenchymal and non-parenchymal cells were isolated from adult rat liver that had been fully regenerated after a 70% partial hepatectomy. The characteristics of the parenchymal cell preparations from regenerated rat liver indicated that they were a homogeneous population and comparable with parenchymal cells isolated from intact liver. The parenchymal cells from regenerated adult rat liver contain glucokinase, hexokinase, pyruvate kinase type I and aldolase B. The non-parenchymal cells contain hexokinase, pyruvate kinase type III and aldolase B. When cells were isolated at different times of the day from rats on controlled feeding schedules, variation of tyrosine aminotransferase activity and liver glycogen content were observed in the parenchymal cells in keeping with the reported diurnal oscillations found in whole liver extracts. When parenchymal cells were isolated from rats 48 and 72h after partial hepatectomy, different isoenzyme patterns were observed. These cells appeared to synthesize pyruvate kinase type III, a function that was assigned previously to non-parenchymal cells or to foetal rat liver hepatocytes.