Catabolism of myo-Inositol to Precursors Utilized for De Novo Glycerolipid Biosynthesis

Systemic injection of [2-3H]myo-inositol into frogs resulted in the incorporation of more than half of the label into glycerolipid classes other than phosphoinositides in retinal rod outer segment membranes. Following methanolysis and differential extraction of isolated lipid classes, radioactivity was recovered primarily in the aqueous phase. After phospholipase C hydrolysis of the total membrane lipids, 97% of the radioactivity was extractable with organic solvents, and 70% of the label in lipids was in 1,2-diglycerides. These results indicate that the label was incorporated primarily into the glyceryl moiety of the membrane glycerolipids. Intraocular injection of frog eyes or in vitro incubation of frog retinas with [2-3H]myo-inositol resulted in the incorporation of radioactivity almost exclusively into phosphoinositides in rod outer segment membranes. Incubation of retinas with [U-14C]glucuronic acid did not result in the formation of labeled retinal lipids. These results suggest that myo-inositol can be catabolized systemically to precursors utilized for glycerolipid biosynthesis in the retina.     

Effect of Electrical Stimulation on Phosphoinositide Metabolism in Rat Sciatic Nerve In Vivo

The metabolism of phosphoinositides in rat sciatic nerves in vivo during electrical stimulation was studied. Nerves were prelabeled by injection of [2-3H]-myo-inositol alone for periods of 2 and 20 h or together with [32P]orthophosphate for 2 h and then electrically stimulated (100 Hz) for 5 or 20 min. Contralateral unstimulated nerve served as the control. When tritiated myo-inositol was used alone for prelabeling the nerves, approximately 6% and 14% of the label was incorporated into lipids after 2 h and 20 h, respectively. Both 5 and 20 min of electrical stimulation caused an insignificant change in the percentage of radioactivity recovered in lipids from the nerves prelabeled with either myo-inositol or with a mixture of myo-inositol and phosphate. The proportion of label associated with phosphoinositides of nerves prelabeled with myo-inositol for both 2 h and 20 h showed an increase in phosphatidyl-inositol-4-phosphate at the expense of phosphatidylinositol in stimulated nerves. Similar results were obtained with nerves prelabeled for 2 h with a mixture of [32P]orthophosphate and [2-3H]myo-inositol. No significant changes in the radioactivity associated with water-soluble inositol phosphates were found in stimulated versus control nerves.     

Rapid transformation of [3H]cholesteryl ester in rat high-density lipoprotein: in vivo and in vitro studies

[24,25-3H]Cholesteryl ester-labeled rat high-density and low-density lipoproteins were administered to recipient rats. Following death of the rats, a major portion of the radioactivity in administered [3H]cholesteryl ester-high-density lipoprotein rapidly appeared in less dense [3H]cholesteryl ester-lipoproteins and was isolated with the low-density lipoprotein fraction. The specific activity of the esterified cholesterol in the product lipoproteins found with the low-density lipoproteins exceeded that of the precursor high-density lipoproteins. In vitro, the addition of [3H]cholesteryl ester-high-density lipoprotein to plasma resulted in a five- to six-fold increase in radioactivity recovered in the low-density lipoprotein. These results demonstrate that, under a variety of experimental conditions, isolated high-density lipoprotein particles (both in vitro and in vivo) tend to become larger and less dense. Rapid changes in the density of lipoproteins labeled with [3H]cholesteryl ester must be considered when interpreting physiologic studies using this label.     

Metabolic fate and effect on cholesterol removal of liposomes prepared from 1,3-di-O-octadecenylglycero-2-phosphocholine studied in vivo and in vitro

Available methodology was adapted to synthesize a labeled diether analog of 2-phosphatidylcholine (1,3-di-O-9'-cis-[9',10' (n)-3H]octadecenylglycero-2-phosphocholine [( 3H]DOE-2-PC). Unilamellar liposomes prepared by sonication from this phospholipid were injected into rats and, 4 h later, 65-78% of injected label was recovered in the liver. Thereafter, liver radioactivity disappeared with a half-life of 2-3 days. The radioactivity lost from the liver was recovered in the feces and in bile. Analysis of liver radioactivity showed that at all time intervals examined (4 h to 3 days after injection), 90% of the label remained as phospholipid. These findings provide evidence that this structural isomer is not readily metabolized, but is fairly rapidly eliminated from the liver. Of the 10% recovered as neutral lipid, 70% comigrated with diacylglycerol and 30% with triacylglycerol. Similar results were obtained when human hepatoma G2 cells in culture were incubated with [3H]DOE-2-PC liposomes. Following incubation of liposomes with liver homogenates, up to 10% conversion of [3H]DOE-2PC to neutral lipid occurred at pH 4.6, but not at pH 7.4. These data show that conversion of [3H]DOE-2-PC to dialkenylglycerol is catalyzed by a lysosomal enzyme. In separate experiments with cultured cells, sonicated dispersions of DOE-2-PC were mixed with high-density apolipoprotein and were shown to enhance markedly cellular cholesterol efflux. This novel diether phospholipid fulfills some of the criteria required of liposomes for their ability to remove cholesterol from the periphery as well as for drug delivery to the liver, i.e., stability in the circulation, marked hepatic uptake, slow metabolism, and elimination from the body.     

Biosynthesis of carnitine and 4-N-trimethylaminobutyrate from lysine

The conversion of l-[U-(14)C]lysine into carnitine was demonstrated in normal, choline-deficient and lysine-deficient rats. In other experiments in vivo radioactivity from l-[4,5-(3)H]lysine and dl-[6-(14)C]lysine was incorporated into carnitine; however, radioactivity from dl-[1-(14)C]lysine and dl-[2-(14)C]lysine was not incorporated. Administered l-[Me-(14)C]methionine labelled only the 4-N-methyl groups whereas lysine did not label these groups. Therefore lysine must be incorporated into the main carbon chain of carnitine. The methylation of lysine by a methionine source to form 6-N-trimethyl-lysine is postulated as an intermediate step in the biosynthesis of carnitine. Radioactive 4-N-trimethylaminobutyrate (butyrobetaine) was recovered from the urine of lysine-deficient rats injected with [U-(14)C]lysine. This lysine-derived label was incorporated only into the butyrate carbon chain. The specific radioactivity of the trimethylaminobutyrate was 12 times that of carnitine isolated from the urine or carcasses of the same animals. These data further support the idea that the last step in the formation of carnitine from lysine was the hydroxylation of trimethylaminobutyric acid, and are consistent with the following sequence: lysine+methionine --> 6-N-trimethyl-lysine --> --> 4-N-trimethylaminobutyrate --> carnitine.     

Primary role of calcium ions in arachidonic acid release from rat platelet membranes. Comparison with human platelet membranes.

The liberation of arachidonic acid (AA) was investigated in platelet membranes prelabelled with [3H]AA. In rat platelet membranes, Ca2+ at concentrations over several hundred nanomolar induced [3H]AA release, with a concurrent decrease in 3H radioactivity of phosphatidylethanolamine and phosphatidylcholine. Some 4-6% of total radioactivity incorporated into platelet membrane lipids was released at 1-10 microM-Ca2+, which is nearly equivalent to that attained in agonist-stimulated platelets. Formation of lysophospholipids in [3H]glycerol-labelled membranes and decrease in [3H]AA liberated by the phospholipase A2 inhibitors mepacrine and ONO-RS-082 suggest that [3H]AA release is mainly catalysed by phospholipase A2. In intact platelets agonist-stimulated [3H]AA release was markedly decreased in the absence of extracellular Ca2+ or in the presence of the intracellular Ca2+ chelator quin 2. These results indicate that in rat platelets the rise of intracellular Ca2+ plays a primary role in the activation of phospholipase A2. In contrast, Ca2+ even at high millimolar concentrations did not effectively stimulate [3H]AA release in human platelet membranes. Thus factor(s) additional to or independent of Ca2+ is required for the liberation of AA in human platelets.     

Membrane morphogenesis in retinal rod outer segments: inhibition by tunicamycin

Isolated Xenopus laevis retinas were incubated with 3H-labeled mannose or leucine in the presence or absence of tunicamycin (TM), a selective inhibitor of dolichyl phosphate-dependent protein glycosylation. At a TM concentration of 20 micrograms/ml, the incorporation of [3H]mannose and [3H]leucine into retinal macromolecules was inhibited by approximately 66 and 12-16%, respectively, relative to controls. Cellular uptake of the radiolabeled substrates was not inhibited at this TM concentration. Polyacrylamide gel electrophoresis revealed that TM had little effect on the incorporation of [3H]leucine into the proteins of whole retinas and that labeling of proteins (especially opsin) in isolated rod outer segment (ROS) membranes was negligible. The incorporation of [3H]mannose into proteins of whole retinas and ROS membranes was nearly abolished in the presence of TM. Autoradiograms of control retinas incubated with either [3H]mannose or [3H]leucine exhibited a discrete concentration of silver grains over ROS basal disc membranes. In TM-treated retinas, the extracellular space between rod inner and outer segments was dilated and filled with numerous heterogeneously size vesicles, which were labeled with [3H]leucine but not with [3H]mannose. ROS disc membranes per se were not labeled in the TM-treated retinas. Quantitative light microscopic autoradiography of retinas pulse-labeled with [3H]leucine showed no differences in labeling of rod cellular compartments in the presence or absence of TM as a function of increasing chase time. These results demonstrate that TM can block retinal protein glycosylation and normal disc membrane assembly under conditions where synthesis and intracellular transport of rod cell proteins (e.g., opsin) are not inhibited.     

Stimulation of phosphoinositide hydrolysis by oxytocin and the mechanism by which oxytocin controls prostaglandin synthesis in the ovine endometrium.

Slices of caruncular endometrium from steroid-treated ovariectomized sheep were incubated with myo-[2-3H]inositol to label tissue phosphatidylinositol. Effects of oxytocin were determined on the rate of incorporation of radioactivity into phosphatidylinositol and on the hydrolysis of phosphoinositides to inositol phosphates and diacylglycerol. Incorporation of radioactivity into phosphatidylinositol was linear during 2 h incubations; 10(-7) M (100 nM)-oxytocin caused a 2.8-fold increase in the rate of incorporation. In the presence of Li+, addition of 10(-7) M-oxytocin to slices in which phosphatidylinositol was pre-labelled caused mean increase of 40-fold in the incorporation of radioactivity into inositol mono-, bis- and tris-phosphates. Inositol 1,3,4-trisphosphate was quantitatively the major trisphosphate formed. The action of oxytocin on phosphoinositide hydrolysis was dose- and time-dependent, occurring at concentrations within the range observed in plasma during episodes of secretion in vivo, and with a time course comparable with that of the action of oxytocin on uterine prostaglandin production. The effect of oxytocin on incorporation of radioactivity into inositol phosphates was not affected by inhibitors of prostaglandin synthesis. Diacylglycerol 1- and 2-lipases in caruncular endometrium converted up to 72% of added 2-[3H]arachidonyldiacylglycerol into [3H]arachidonic acid during 30 min incubations at pH 7.0. Caruncular endometrium contained 1.49 mumol of phosphatidylinositol/g, representing approx. 0.2 mumol/g of phosphatidylinositol arachidonic acid. It is proposed that the stimulation of endometrial prostaglandin synthesis by oxytocin is accounted for by increased hydrolysis of phosphoinositides to diacylglycerol and inositol phosphates with subsequent release of arachidonic acid from diacylglycerol.     

Generation of Arachidonic Acid and Diacylglycerol Second Messengers from Polyphosphoinositides in Ischemic Fetal Brain

Intracerebral administration of [3H]arachidonic acid ([3H]ArA) into 19-20-day-old rat embryos, resulted in a rapid incorporation of label into brain lipids. One hour after injection, 55.6 +/- 8.2, 18.0 +/- 3.4, and 13.7 +/- 1.3% of the total radioactivity was associated with phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine, respectively. Approximately 10% of radioactivity was found acylated in neutral lipids of which free ArA comprised only 1.5 +/- 0.2% of the total radioactivity. Complete restriction of the maternal-fetal circulation for < or = 40 min did not affect the rate of [3H]ArA incorporation (t1/2 = 2 min) into fetal brain lipids, suggesting an effective acylation mechanism that proceeds irrespective of the impaired blood flow. After a short restriction period (5 min), the radioactivity in diacylglycerol was elevated by 50%. After a longer restriction period (20 min), the radioactivity in the free fatty acid and diacylglycerol fractions increased to values of 130 and 87%, respectively. Polyphosphoinositides prelabeled with either [3H]ArA or 32P were rapidly degraded after 5 min of ischemia. After 20 min, the decrease in phosphatidylinositol-4-phosphate and phosphatidylinositol-4,5-bisphosphate radioactivity was 47 and 70%, respectively. Double labeling of phospholipids with [14C]palmitic acid and [3H]ArA indicated a preferential loss of [3H]ArA within the polyphosphoinositide species after 20 min, but not after 5 min of ischemia. The specific activity of [14C]palmitate remained unchanged. The current data suggest phospholipase C-mediated diacylglycerol formation at the beginning of the insult followed by a phospholipase A2-mediated ArA liberation at a later time, both enzymes presumably acting preferentially on polyphosphoinositide species.     

Methylated messenger RNA in mouse kidney.

Polyadenylated messenger RNA from mouse kidney labeled in vivo exhibited a pattern of methylation distinct from that of rRNA and tRNA. After mice were given L-[methyl-3H]methionine, 4% of the polyribosomal RNA label was bound to oligo (dT)-cellulose; 20-24% of orotate- or adenine-labeled polyribosomal RNA eluted in the poly(A)+ RNA fraction under similar conditions. [3H]Methyl radioactivity was not incorporated into low molecular weight (5-5.8 S) rRNA, indicating the extent of nonmethylpurine ring labeling was negligible. [3H]Methyl-labeled poly(A)+ RNA sedimented heterogeneously in sodium dodecyl sulfate containing gradients similarly to poly(A)+ mRNA labeled with [3H]orotic acid. Based on an average molecular length of 2970 nucleotides, renal mRNA was estimated to contain 8.6 methyl moieties per molecule. Analysis of alkaline-hydrolyzed RNA sampled by DEAE-Sephadex-urea chromatography provided estimates of the relative amounts of base and ribose methylation. Although 83% of the [3H]methyl radioactivity in rRNA was in the 2'-0-methylnucleotide fraction, no methylated dinucleotides were found in mRNA. In poly(A)+ mRNA 60% of the [3H]methyl label was in the mononucleotide fraction; the remainder eluted between the trinucleotide and tetranucleotide markers and had a net negative charge between -4 and -5. The larger structure, not yet charcterized, could result from two or three consecutive 2'-0-ribose methylations and is estimated to contain 2.6 methyl residues. Alternatively, the oligonucleotide could be a 5'-terminal methylated nucleotide species containing 5'-phosphate(s) in addition to the 3'-phosphate moiety resulting from alkaline hydrolysis. Either structure could have a role in the processing or translation of mRNA in mammalian cells.     

Sialic acid incorporation into gangliosides and glycoproteins of the fish brain

—The concentration of lipid- and non-lipid-bound sialic acid in the optic nerve tract and tectum and in whole brain of fish was estimated. The incorporation of sialic acid into gangliosides and non-lipid components was studied in fish by intracranial or intraocular application of N-[³H]acetylmannosamine or N-[³H]acetylglucosamine. After intracranial injection of N-[³H]acetylmannosamine autoradiography showed lipid- and non-lipid-bound radioactivity in the tectum opticum evenly distributed over regions of nerve fibres or perikarya indicating an ubiquitous incorporation of label. Sialic acid incorporation into glycoproteins after intracranial injection of N-acetylmannosamine always exceeded that into gangliosides. TCA-precipitable non-lipid material is labelled from intracranially applied N-acetylmannosamine in the sialic acid portion and also in nonsialic acid components, whereby the percentage of label in sialic acid increases reaching 90 per cent of the total radioactivity after 90 min. After intraocular application of N-[³H]acetylmannosamine, sialic acid in gangliosides was generally found to be more highly labelled than in glycoproteins. The ratio of radioactivity in gangliosides and glycoproteins increased with time of incubation and the distance from the eye. TCA-soluble radioactivity was translocated by fast axonal transport. Cycloheximide inhibited incorporation of N-acetylmannosamine-derived radioactivity into gangliosides and proteins but not the transport of TCA-soluble material, which accumulates in the tectum. After intraocular application of N-[³H]acetylglucosamine, TCA-soluble label arrives later in the optic tectum than radioactivity of high molecular weight components. The ratio of lipid to non-lipid-bound radioactivity does not change considerably with the time after injection or the distance from the eye. There was no accumulation of TCA-soluble radioactivity after the inhibition of incorporation into high molecular weight components.     

Extracellular hydrolysis of formyl peptides and subsequent uptake of liberated amino acids by alveolar macrophages

The mechanism of accumulation of radioactive label from fNle-Leu-[3H]Phe by guinea pig alveolar macrophages was investigated. The binding of fNle-Leu-[3H]Phe to macrophages reached equilibrium within 5 min at 4 degrees C, but equilibrium could not be achieved at temperatures where fNle-Leu-Phe stimulated superoxide anion production is observed (e.g., 21-23 degrees C). At this temperature a rapid phase of initial binding of fNle-Leu-[3H]Phe to its receptor was followed by continued accumulation of cell-associated radioactivity which was linear and was dependent on the extracellular pH, i.e., the rate increased as the pH was lowered from pH 8 to pH 6. Examination for possible intracellular hydrolysis of fNle-Leu-[3H]Phe revealed the presence of extensive amounts of [3H]phenylalanine, both cell-associated and in the medium. The increases in cell-associated [3H]phenylalanine correlated in time and pH with cell-associated radioactivity that was accumulated after stimulation with fNle-Leu-[3H]Phe. The addition of 1 mM unlabelled phenylalanine blocked the long term accumulation of label from fNle-Leu-[3H]Phe by macrophages. 1 mM phenylalanine had no measureable effect on fNle-Leu-Phe stimulated O2- production, fNle-Leu-[3H]Phe hydrolysis or on fNle-Leu-[3H]Phe binding to its receptor. These results indicated that the long term accumulation of radioactivity by alveolar macrophages was due to extracellular hydrolysis of fNle-Leu-[3H]Phe followed by transport of liberated [3H]phenylalanine into the cells. A high affinity (Km = 3.56 X 10(-8) M) transport system for phenylalanine was measured in alveolar macrophages, which was not stimulated by the addition of fNle-Leu-Phe. The extracellular hydrolysis of fNle-Leu-[3H]Phe could not be attributed to release of macrophage enzymes into the medium. The responsible proteinase appears to be membrane bound and has a Km for the hydrolysis of fNle-Leu-[3H]Phe of 2.6 X 10(-7) M which is similar to the Kd (1.5 X 10(-7) M) for fNle-Leu-Phe binding. Taken together, these data suggest that for the alveolar macrophage: (1) formyl peptides are not internalized by a receptor-mediated process; (2) a surface proteinase can catalyze the hydrolysis of formyl peptides; and (3) [3H]phenylalanine formed by fNle-Leu-[3H]Phe hydrolysis is transported into the interior of the macrophage.     

Entry of Newly Synthesized Gangliosides into Myelin

Ganglioside synthesis and transport to myelin was studied in brainstem slices prepared from 19-21-day-old rats. The slices were incubated for up to 2 h in the presence of [3H]glucosamine to label primarily the hexosamine portion of complex gangliosides. The amount of radioactivity incorporated into gangliosides during slice incubations was only 10-15% of the amount of the label incorporated during in vivo labeling of brainstem gangliosides using equivalent amounts of [3H]glucosamine. Among individual gangliosides this inhibition was greater for the more complex gangliosides. When labeled gangliosides were isolated from homogenate and myelin fractions prepared from brain slices, the complex total gangliosides of both fractions showed a lag in labeling kinetics but with a lower specific radioactivity for the myelin fraction, reflecting the larger pool size and slower turnover rate exhibited by myelin components. Chase experiments showed that more complex gangliosides in homogenate exhibited almost no effect of chase after 30 min. Addition of the Golgi-disrupting agent monensin to slice incubations inhibited the labeling of all gangliosides except GM3, GM2, and GD3, and transport to myelin of all complex gangliosides except GM2. These results show that a monensin-sensitive mode of transport is responsible for the translocation of most newly synthesized gangliosides into myelin.     

Subcellular location and identification of a large molecular weight substrate for the liver plasma membrane transglutaminase

When the particulate fraction from a rat liver homogenate was incubated with [3H]putrescine and calcium, the radioactive amine was incorporated into the membranes via a transglutaminase-mediated reaction. Fractionation of the membranes by isopycnic density gradient centrifugation revealed that the radioactive label was coincident with the 5'-nucleotidase and transglutaminase activities which serve as markers for the plasma membrane (Slife, C. W., Dorsett, M. D., Bouquett, G. T., Register, A., Taylor, E., and Conroy, S. Arch. Biochem. Biophys. 241, 329-336). If the labeled membranes were treated with digitonin and fractionated, the radioactivity and the plasma membrane enzyme activities coincidentally shifted to a greater density. Examination of the [3H]putrescine-labeled membranes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography showed that the largest amount of radioactivity was associated with a large molecular weight material that did not enter the acrylamide gel. Pulse-chase experiments indicated that the large aggregate already was present in the native membrane, or that it was formed very rapidly during the putrescine incubation. The complex did not result from putrescine cross-linking between proteins since dansylcadaverine and [3H]histamine were also selectively incorporated into it. These data show that there are protein substrates in the plasma membrane which are accessible to the membrane-associated transglutaminase and that the substrates form a large molecular weight aggregate which is not dissociated by sodium dodecyl sulfate and disulfide reducing agents.     

A novel labeling procedure of anteiso-fatty acid-containing lipids in staphylococci for investigating the effect of penicillin on lipid release

L-[4,5-3H]isoleucine was introduced to label anteiso-fatty acid (AIFA)-containing lipids in Staphylococcus aureus SG 511. After an overnight incubation in peptone broth in the presence of 37 kBq L-[4,5-3H]isoleucine/ml, 8.5-13% of the total radioactivity applied was found to be incorporated into the cells. 22.4-25.6% of the incorporated radioactivity was found in AIFA-containing lipids extracted by chloroform-methanol-water (2:1:0.2, v/v/v) at pH 2. The interphase contained 70-75% of the incorporated radioactivity. Lipoteichoic acid, extracted by phenol-water (80:20, w/v) contained less than 1% of the incorporated radioactivity, as measured after purification by hydrophobic interaction chromatography on octyl sepharose gel. Within 1 h after addition of 10 micrograms/ml penicillin G to exponentially growing cultures of S. aureus, that led to non-lytic death of the cells, 11.9-18.1% of the incorporated L-[4,5-3H]isoleucine label were released. Lipids containing AIFA were excreted to 5.4-8.4% of total incorporated activity; this amount represents more than 1/4 of the labeled cellular lipids.     

Studies on the Secretion of Maize Root Cap Slime: IV. Evidence for the Involvement of Dictyosomes

The involvement of dictyosomes and their vesicles in secretion of slime by maize root cap cells is demonstrated by kinetic and organelle fractionation experiments using l-fucose as a specific marker for the secreted slime. Pulse-chase experiments show that l-[1-(3)H]fucose is incorporated into two distinct fractions of root cap cells. Incorporation into a water-soluble, ethyl alcohol-insoluble fraction of the homogenate has a peak at 20 minutes of chasing followed by rapid loss of label. Seventy per cent of the radioactivity in this fraction is secreted from the tissue during a 2-hour chase period. Incorporation of label from [(3)H]fucose into a water-insoluble fraction is kinetically different suggesting that in situ incorporation of label is occurring into the cell wall. Labeling of the water-soluble, ethyl alcohol-insoluble fraction with an (14)C-amino acid mixture differs from that of [(3)H]fucose. Thus, while release of the [(3)H]fucose-containing polymer begins after 10 to 15 minutes of chasing, the release of the (14)C-amino acid polymer is delayed an additional 5 to 10 minutes and occurs at a lower rate. Cesium chloride density gradient centrifugation of secreted material labeled with radioactivity from [(3)H]fucose indicates the presence of only one major component having a buoyant density similar to that of purified root cap slime (1.63 g cm(-3)). Sucrose density gradient centrifugation of homogenates of [(3)H]fucose-labeled root cap tissue shows that radioactivity in nondialyzable material occurs as a broad band between densities 1.12 and 1.18 g cm(-3) with a peak at density 1.15 g cm(-3), the same density at which dictyosomes were localized by electron microscopy. Autoradiography of organelle fractions shows that radioactivity was associated almost exclusively with dictyosomes.     

Biosynthesis of N-Acetyldopamine and N-Acetyloctopamine by Schistocerca gregaria Nervous Tissue

N-Acetyltyramine, N-acetyldopamine and N-acetyloctopamine were the major products when either L-[3H]tyrosine or [3H]tyramine were incubated with thoracic ganglia of the desert locust, Schistocerca gregaria. No label was incorporated into L-DOPA under these conditions, although 2-3% of the radioactivity could be recovered in dopamine and octopamine. Addition of the aromatic amino acid decarboxylase inhibitor, 3-hydroxybenzylhydrazine (NSD 1015), prevented the formation of N-acetylcompounds from L-[3H]tyrosine, without resulting in an accumulation of label in L-DOPA. In contrast, incubation of samples of haemolymph with L-[3H]tyrosine resulted in the recovery of 7% of label in L-DOPA, which was increased to 17% in the presence of NSD 1015. These results provide evidence that the initial step in the synthesis of dopamine and octopamine by S. gregaria nervous tissue is the conversion of L-tyrosine to tyramine, which is subsequently metabolised to N-acetyltyramine, N-acetyldopamine or N-acetyloctopamine.     

Alterations in nicotinamide and adenine nucleotide systems during mixed-function oxidation of p-nitroanisole in perfused livers from normal and phenobarbital-treated rats

When rat liver nuclei were incubated with [adenine-3H]NAD, besides histone 1, histone 2A and especially histone 2B accepted 3H radioactivity. 3H radioactivity was also found on the non-histone proteins and on the small amounts of histones 1 and 3 released into the supernatant during incubation. [14C]Adenine uptake in vivo by liver and thymus nuclei showed radioactivity in histones 1 and 3. After digestion with Pronase and leucine aminopeptidase 14C- or 32P-labelled histone 3 released a serine phosphate-containing nucleotide, which on acid hydrolysis yielded ADP-ribose and serine phosphate. Serine phosphate was also found in the material from the nucleotide peaks from histones 2A and 2B. ADP-ribosylated histones 1 and 3 were more easily released from nuclei than their unmodified forms and showed higher [32P]Pi and [3H]lysine uptakes in vivo [Ord & Stocken (1975) FEBS Meet. Proc. 34, 113-125].     

Metabolism of [11-3H]retinyl acetate in liver tissues of vitamin A-sufficient, -deficient and retinoic acid-supplemented rats

A study was conducted on the incorporation of [11-3H]retinyl acetate into various retinyl esters in liver tissues of rats either vitamin A-sufficient, vitamin A-deficient or vitamin A-deficient and maintained on retinoic acid. Further, the metabolism of [11-3H]retinyl acetate to polar metabolites in liver tissues of these three groups of animals was investigated. Retinol metabolites were analyzed by high-performance liquid chromatography. In vitamin A-sufficient rat liver, the incorporation of radioactivity into retinyl palmitate and stearate was observed at 0.25 h after the injection of the label. The label was further detected in retinyl laurate, myristate, palmitoleate, linoleate, pentadecanoate and heptadecanoate 3 h after the injection. The specific radioactivities (dpm/nmol) of all retinyl esters increased with time. However, the rate of increase in the specific radioactivity of retinyl laurate was found to be significantly higher (66-fold) than that of retinyl palmitate 24 h after the injection of the label. 7 days after the injection of the label, the specific radioactivity between different retinyl esters were found to be similar, indicating that newly dosed labelled vitamin A had now mixed uniformly with the endogenous pool of vitamin A in the liver. The esterification of labelled retinol was not detected in liver tissues of vitamin A-deficient or retinoic acid-supplemented rats at any of the time point studied. Among the polar metabolites analyzed, the formation of [3H]retinoic acid from [3H]retinyl acetate was found only in vitamin A-deficient rat liver 24 h after the injection of the label. A new polar metabolite of retinol (RM) was detected in liver of the three groups of animals. The formation of 3H-labelled metabolite RM from [3H]retinyl acetate was not detected until 7 days after the injection of the label in the vitamin A-sufficient rat liver, suggesting that metabolite RM could be derived from a more stable pool of vitamin A.     

Subcellular distribution of alpha 1-adrenergic receptors in rat liver

The distribution of alpha 1-adrenergic receptors in rat liver subcellular fractions was studied using the alpha 1-adrenergic receptor ligand [3H]prazosin. The highest number of [3H]prazosin binding sites was found in a plasma membrane fraction followed by 2 Golgi and a residual microsomal fraction, the numbers of binding sites were 1145, 845, 629 and 223 fmol/mg protein, respectively. When the binding in these fractions was compared with the activity of plasma membrane 'marker' enzymes in the same fractions a relative enrichment of [3H]prazosin binding sites was found in the residual microsomes and one of the Golgi fractions. Photoaffinity labelling with 125I-arylazidoprazosin in combination with SDS-polyacrylamide gel electrophoresis revealed the specific binding to 40 and 23 kDa entities in a Golgi fraction, while in plasma membranes the binders had an apparent molecular mass of 36 and 23 kDa. When [3H]prazosin was injected in vivo into rat portal blood followed by subcellular fractionation of liver, a pattern of an initial rapid decline and thereafter a slow decline of radioactivity was noted in all fractions. Additionally, in the two Golgi fractions a transient accumulation of radioactivity occurred between 5 and 10 min after the injection. The ED50 values for displacement of [3H]prazosin with adrenaline was lowest in the plasma membrane fraction, followed by the residual microsomes and Golgi fractions, the values were 10(-6), 10(-5) and 10(-4) mol/l, respectively. On the basis of lack of correlation between distribution of alpha 1-adrenergic antagonist binding and adenylate cyclase activity, differences in the molecular mass of alpha 1-adrenergic antagonist binders, differences in the kinetics of in vivo binding and accumulation of [3H]prazosin and also differences in agonist affinity between plasma membrane and Golgi fractions, it is concluded that alpha 1-adrenergic receptors are localized to low-density intracellular membranes involved in receptor biosynthesis and endocytosis.