Evidence for the role of pancreatic acinar cells in the production of ornithine and guanidinoacetic acid by L-arginine:glycine amidinotransferase.

L-Arginine:glycine amidinotransferase (transamidinase) occurs at high concentrations in the kidney and the pancreas of rats. The cellular localization of transamidinase was investigated in fetal, neonatal, and adult rat pancreatic tissue using three indicators of the presence of transamidinase: (1) immunofluorescence microscopy, (2) in vitro enzymatic activity measurements on homogenates of whole pancreas and on isolated acinar and islet tissue from adult rats, and (3) ornithine production from perfused adult rat pancreas. The cellular localization of transamidinase was determined in fetal, neonatal, and adult rat pancreas, using a polyclonal guinea pig antibody made against a highly purified preparation of kidney transamidinase. Immunoreactive transamidinase was detected only in the pancreatic acinar cells. The cellular distribution of the immunostaining was compatible with the presence of transamidinase in mitochondria. The transamidinase enzymatic activity of whole pancreatic homogenates was 13.4 +/- 0.7 U/g wet weight (n = 11). In pancreata where islets had been isolated away from the acinar tissue, the transamidinase activity was similar to that of the whole pancreatic homogenates (16.8 +/- 2 U/g wet weight). Any transamidinase activity present in isolated islets was below the sensitivity of the assay. Transamidinase activity in the isolated perfused pancreas was determined by measuring the amount of ornithine released into the perfusate. The transamidinase activity of the perfused pancreas was 16.4 +/- 1.8 U/g pancreas and is an estimate of the physiological production capacity of the enzyme (270 +/- 29 nmol ornithine/min/g pancreas). These results indicate that transamidinase is present at high concentrations in the pancreas.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and sequencing of rat kidney L-arginine:glycine amidinotransferase. Studies on the mechanism of regulation by growth hormone and creatine.

L-Arginine-glycine amidinotransferase (transamidinase) is the first and rate-limiting step in creatine biosynthesis. Rats fed a creatine-supplemented diet or hypophysectomized rats have only 20% of the kidney transamidinase activity as intact rats fed a creatine-free diet. A cDNA clone corresponding to transamidinase was isolated by immunoscreening of a lambda gt11 expression library prepared from rat kidney mRNA. The transamidinase cDNA had an open reading frame containing the known sequence of the amino-terminal peptide of transamidinase. Based on the cDNA sequence, transamidinase is synthesized as a precursor with an amino-terminal extension of 50 amino acids, consistent with its mitochondrial localization. Comparison of the transamidinase sequence with the protein data base identified only a single, related protein. Remarkably, this protein, which has a 37% amino acid identity with transamidinase, is also an amidinotransferase, catalyzing streptomycin biosynthesis in Streptomyces griseus. Transamidinase cDNA was used to investigate the regulation of mRNA levels by creatine and growth hormone. Hypophysectomized rats were fed a creatine-free or a creatine-supplemented diet and maintained with and without injections of growth hormone. An excellent correlation was found between changes in transamidinase activity and mRNA levels in response to creatine and growth hormone. Thus, the regulation of transamidinase by creatine and growth hormone is at a pretranslational level. In addition, the two effectors do not act independently but interact at a pretranslational level to control transamidinase gene expression.

Multiple forms of rat kidney L-arginine:glycine amidinotransferase.

The relative amount of L-arginine:glycine amidinotransferase (transamidinase) protein in kidneys from rats fed a complete purified diet with and without the addition of creatine and/or glycine was determined by a monoclonal antibody-immunosorbent inhibition assay. Kidneys from the creatine-fed rats had 10% of the transamidinase activities and 78% of the monoclonal antibody immunoreactive transamidinase protein as kidneys from the control rats. An excellent correlation between transamidinase activities and protein was reported previously when the amounts of enzyme protein were determined by immunotitration with polyclonal antibodies. One possible explanation for the contrasting results was that multiple forms of transamidinase are present in rat kidneys. If so, the monoclonal antibody may have recognized forms of the enzyme that were not decreased in amounts commensurate with the decrease in enzyme activities as a result of creatine feeding. Evidence is presented in this report that multiple forms of transamidinase are present in rat kidneys. The distribution of the isoelectric points of the individual forms of transamidinase in kidneys of the control rats appeared to be dissimilar from that in the creatine-fed rats. Therefore, an alteration in the distribution of the individual forms of the enzyme may be a factor in the alteration of transamidinase activities in creatine-fed rats.

Localization of L-arginine-glycine amidinotransferase protein in rat tissues by immunofluorescence microscopy.

Creatine is a major component of energy metabolism and enzymes involved in its synthesis have therefore been of considerable interest. L-arginine-glycine amidinotransferase, commonly called transamidinase, catalyzes the first reaction in the biosynthesis of creatine. This first reaction is believed to occur in the kidney because of the high concentration of transamidinase in that tissue. Transamidinase activity is also found in many other tissues of the rat, but its role in these tissues is not known. Immunochemical studies with antisera and monoclonal antibodies were used to confirm and refine our understanding of the presence of transamidinase in rat tissues. Immunofluorescence histochemistry was performed to localize transamidinase immunoreactivity within specific tissues including cells in the proximal tubules of the kidney, hepatocytes of the liver, and alpha cells of the pancreatic islet. Immunochemical studies with monoclonal antibodies confirm localization of transamidinase immunoreactivity in the proximal tubules of the kidney. The localization of such immunoreactivity in specialized cells yields insight into possible physiological role(s) of transamidinase in the rat.

Role of sterol carrier protein in cholesterol metabolism.

This report summarizes our recent studies on the protein known as sterol carrier protein (SCP) or fatty acid binding protein (FABP). SCP is a highly abundant, ubiquitous protein with multifunctional roles in the regulation of lipid metabolism and transport. SCP in vitro activates membrane-bound enzymes catalyzing cholesterol synthesis and metabolism, as well as those catalyzing long chain fatty acid metabolism. SCP also binds cholesterol and fatty acids with high affinity and rapidly penetrates cholesterol containing model membranes. Studies in vivo showed SCP undergoes a remarkable diurnal cycle in level and synthesis, induced by hormones and regulated in liver by translational events. SCP rapidly responds in vivo to physiological events and manipulations affecting lipid metabolism by changes in level. Thus SCP appears to be an important regulator of lipid metabolism. Preliminary evidence is presented that SCP is secreted by liver and intestine into blood and then taken up by tissues requiring SCP but incapable of adequate SCP synthesis.

Translational control of the circadian rhythm of liver sterol carrier protein. Analysis of mRNA sequences with a specific cDNA probe.

The striking changes in amount of rat liver SCP (sterol carrier protein) during a 24-h dark-light cycle are due to alterations in the relative synthetic rate of SCP. However, functional SCP mRNA, measured by a cell-free translational assay, does not fluctuate in the dark-light cycle. Since cell-free translational assays do not always reflect the actual abundance of an mRNA molecule, a specific cDNA hybridization assay was used to directly quantitate SCP mRNA sequences. The cDNA probe was selected from a rat liver library by hybridization to a mixture of synthetic oligonucleotides containing a portion of the sequence of SCP mRNA. The relative amount and size distribution of the SCP mRNA species (approximately 700-800 nucleotides) does not change during the diurnal cycle. To explore possible mechanisms of this translational control, the polysomal distribution of SCP mRNA was compared at the maximum and minimum points of SCP synthesis. No significant amounts of SCP mRNA were present in nonpolysomal ribonucleoprotein particles. Furthermore, no alteration in the relative level of SCP mRNA associated with polysomes or in polysome size occurs at the maximum and minimum points of SCP synthesis. Thus, changes in total SCP mRNA levels or its polysomal distribution cannot account for the diurnal variation in SCP synthesis.

The production and characterization of two monoclonal antibodies to rat kidney L-arginine: glycine amidinotransferase.

Rat kidney L-arginine:glycine amidinotransferase (transamidinase) has been purified previously to homogeneity as two fractions, designated alpha and beta. No differences in the properties of these two fractions could be found. Two monoclonal antibodies (Tran/NS-1/1 and Tran/NS-1/3) to the purified alpha fraction of rat kidney transamidinase were produced, purified, and characterized. The results of competitive binding studies of the two monoclonal antibodies to alpha transamidinase were as follows: 1) Tran/NS-1/3 had no effect on 125I-Tran/NS-1/1 binding while Tran/NS-1/1 inhibited 125I-Tran/NS-1/1 binding; 2) Tran/NS-1/3 inhibited 125I-Tran/NS-1/3 binding while Tran/NS-1/1 had no effect on 125I-Tran/NS-1/3 binding. Therefore, Tran/NS-1/1 and Tran/NS-1/3 bound to different antigenic determinants on alpha transamidinase. 125I-Tran/NS-1/1 and 125I-Tran/NS-1/3 each had high avidity constants (approximately 10(7)-10(9)) for both alpha and beta rat kidney transamidinase. Tran/NS-1/1 and Tran/NS-1/3 bound to human kidney transamidinase in ELISA assays. A quantitative immunosorbent inhibition assay for rat kidney transamidinase was developed with 125I-Tran/NS-1/3. Approximately 30 ng of immunoreactive transamidinase could be detected by this immunosorbent inhibition assay. The amount of Tran/NS-1/3 immunoreactive species in rat lung and testicular tissue by the immunosorbent inhibition assay correlated well with the amount of transamidinase activity found in those tissues. The availability of the monoclonal antibodies, Tran/NS-1/1 and Tran/NS-1/3, should facilitate studies of rat and human transamidinase structure and regulation.

Repression of rat kidney L-arginine:glycine amidinotransferase synthesis by creatine at a pretranslational level.

The first committed reaction in the biosynthesis of creatine is catalyzed by the enzyme L-arginine:glycine amidinotransferase, commonly called transamidinase. Creatine, the end product of the biosynthetic pathway, is known to alter the levels of kidney transamidinase activity. Rats fed a diet containing 0.3% creatine had 26% of the kidney transamidinase activity of the rats fed a creatine-free diet. This reduction in transamidinase activity was correlated with a decrease in transamidinase protein in the creatine-fed rats. The relative synthetic rates and mRNA functional activities of transmidinase were measured in control and creatine-fed rats. The relative synthetic rate of transamidinase in creatine-fed rats was 21% of that found in the control animals. The functional transamidinase mRNA in creatine-fed rats was correspondingly reduced to 37% of the amount in the control animals. Thus, creatine affects transamidinase activity by altering its rate of synthesis at a pretranslational step and represents an example of end-product repression in a higher eukaryote.

Translational control of the circadian rhythm of liver sterol carrier protein.

Rat liver sterol carrier protein (SCP), a major regulator of lipid metabolism and transport, undergoes a rapid turnover and dramatic circadian variation in amount. The level of SCP was quantitated by a specific immunochemical assay using an antibody to homogeneous liver SCP. During a 12-h dark, 12-h light cycle, liver exhibits a biphasic pattern in SCP level. A 7-fold increase in SCP (i.e. from 1 to 7 mg/g of liver) occurs in the dark period, peaking at the midpoint and returning to basal levels by the beginning of the light period. A similar but smaller pattern of variation in SCP amount occurs in the light cycle. To elucidate the basic mechanism responsible for these changes in SCP level, the relative synthetic rate of SCP and mRNA functional activity for SCP were measured during the dark-light cycle. Alterations in the rate of SCP synthesis can account for the variations in SCP concentration. Although large changes occur in relative synthetic rate, no significant changes were found in the level of mRNA for SCP. Therefore, the circadian rhythm in SCP synthesis and amount does not reflect variations in the concentration of mRNA for SCP, but instead is caused by some mechanism controlling the efficiency of translation of SCP mRNA.