Muon spectroscopy applied to biological systems: a study of thiyl radicals, RS

Thiyl radicals (RS.) are formed when positive muons (mu+) are implanted into solutions of thiocarbonyl compounds (C = S) in ethanol, tetrahydrofuran or formamide solvents. A solvent dependence is found, which reflects changing electronic interactions and overall conformations, suggesting that the properties of RS. radicals may be dependent on the polarity (philicity) of the environment in which they are formed.

Epidemic thresholds and vaccination in a lattice model of disease spread

We use a lattice-based epidemic model to study the spatial and temporal rates of disease spread in a spatially distributed host population. The prevalence of the disease in the population is studied as well as the spread of infection about a point source of infection. In particular, two distinct critical population densities are identified. The first relates to the minimum population density for a epidemic to occur, whilst the second is the minimum population density for long-term persistence to occur. Vaccination regimes are introduced that are used to measure the impact of spatially and nonspatially dependent intervention strategies. Specifically we show how a ring of vaccinated susceptibles, of sufficient thickness, can halt the spread of infection across space.

Multicomponent spectroscopic investigations of salivary antioxidant consumption by an oral rinse preparation containing the stable free radical species chlorine dioxide (ClO2.)

A multicomponent evaluation of the oxidative consumption of salivary biomolecules by a commercially-available oral rinse preparation containing an admixture of the stable free radical species chlorine dioxide (ClO2.) with chlorite anion (ClO2-) has been investigated using high resolution 1H NMR spectroscopy. The results obtained demonstrated that ClO2. and/or ClO2- present in this preparation effected the oxidative decarboxylation of salivary pyruvate (to acetate and CO2). Experiments conducted on chemical model systems confirmed the oxidative decarboxylation of pyruvate by this oral rinse, and also demonstrated that urate, thiocyanate anion, and the amino acids cysteine and methionine (precursors to volatile sulphur compounds responsible for oral malodour), were oxidatively consumed. The biochemical, periodontal and therapeutic significance of the results are discussed.

The kinetics of immunological responses to microfilariae in a murine host: experimental and mathematical studies

We present a mathematical model which is used to interpret the dynamics of the immunological response of a mouse host to infection with the filarial worm Onchocerca lienalis. The model mimics changes in worm burden over time post-infection and after reinfection and its behaviour provides a good description of experimental results. Measured production of T-cells and eosinophils is also compared with the predictions of the model. Our results show that the immune response mechanism proposed on the basis of experimental results, involving CD4+ T-cells and eosinophil destruction of the parasite, is supported by the insights gained from the mathematical model. Also, using the parameters estimated to describe the primary infection dynamics, the degree of acquired immunity to secondary infection is also well described by the model. Our analysis highlights the importance of obtaining quantitative measures of the many rate parameters involved in even the simplest interpretations of immunological responses to parasitic infection.

A scaling analysis of measles epidemics in a small population

We present a detailed analysis of the pattern of measles outbreaks in the small isolated community of the Faroe Islands. Measles outbreaks in this population are characterized by frequent fade-out of infection resulting in long intervals when the disease is absent from the islands. Using an analysis of the distribution of epidemic sizes and epidemic durations we propose that the dynamical structure observed in the measles case returns reflects the existence of an underlying scaling mechanism. Consequently the dynamics are not as purely stochastic as is usually thought for epidemiological systems of this sort. We use a lattice-based epidemic model to provide a theoretical estimate of the scaling exponents and show that a conventional compartmental SEIR model is unable to reproduce this result. The methods discussed in this paper are general and represent a novel way to consider the dynamics of any other communicable disease where there is frequent fade-out in the case returns.

Insulin-stimulated translocation of GLUT4 glucose transporters requires SNARE-complex proteins

A major physiological role of insulin is the regulation of glucose uptake into skeletal and cardiac muscle and adipose tissue, mediated by an insulin-stimulated translocation of GLUT4 glucose transporters from an intracellular vesicular pool to the plasma membrane. This process is similar to the regulated docking and fusion of vesicles in neuroendocrine cells, a process that involves SNARE-complex proteins. Recently, several SNARE proteins were found in adipocytes: vesicle-associated membrane protein (VAMP-2), its related homologue cellubrevin, and syntaxin-4. In this report we show that treatment of permeabilized 3T3-L1 adipocytes with botulinum neurotoxin D, which selectively cleaves VAMP-2 and cellubrevin, inhibited the ability of insulin to stimulate translocation of GLUT4 vesicles to the plasma membrane. Furthermore, treatment of the permeabilized adipocytes with glutathione S-transferase fusion proteins encoding soluble forms of VAMP-2 or syntaxin-4 also effectively blocked insulin-regulated GLUT4 translocation. These results provide evidence of a functional role for SNARE-complex proteins in insulin-stimulated glucose uptake and suggest that adipocytes utilize a mechanism of regulating vesicle docking and fusion analogous to that found in neuroendocrine tissues.

Insulin-secreting non-islet cells are resistant to autoimmune destruction

Transgenic nonobese diabetic mice were created in which insulin expression was targeted to proopiomelanocortin-expressing pituitary cells. Proopiomelanocortin-expressing intermediate lobe pituitary cells efficiently secrete fully processed, mature insulin via a regulated secretory pathway, similar to islet beta cells. However, in contrast to the insulin-producing islet beta cells, the insulin-producing intermediate lobe pituitaries are not targeted or destroyed by cells of the immune system. Transplantation of the transgenic intermediate lobe tissues into diabetic nonobese diabetic mice resulted in the restoration of near-normoglycemia and the reversal of diabetic symptoms. The absence of autoimmunity in intermediate lobe pituitary cells engineered to secrete bona fide insulin raises the potential of these cell types for beta-cell replacement therapy for the treatment of insulin-dependent diabetes mellitus.

Power laws governing epidemics in isolated populations

Temporal changes in the incidence of measles virus infection within large urban communities in the developed world have been the focus of much discussion in the context of the identification and analysis of nonlinear and chaotic patterns in biological time series. In contrast, the measles records for small isolated island populations are highly irregular, because of frequent fade-outs of infection, and traditional analysis does not yield useful insight. Here we use measurements of the distribution of epidemic sizes and duration to show that regularities in the dynamics of such systems do become apparent. Specifically, these biological systems are characterized by well-defined power laws in a manner reminiscent of other nonlinear, spatially extended dynamical systems in the physical sciences. We further show that the observed power-law exponents are well described by a simple lattice-based model which reflects the social interaction between individual hosts.

Persistence and dynamics in lattice models of epidemic spread

We present a simple epidemic model representing the spread of a communicable disease in a spatially extended host population. The model falls into the general class of techniques which utilise lattice based simulation as a way of incorporating spatial effects. The factors relating to the persistence and dynamics of the disease are investigated. There exists a clear population threshold below which the disease dies out and above which it settles to an endemically stable state. The rate of population mixing is shown to affect this threshold density. Equations which accurately account for the mean-field limit of the model are introduced and the relevance to the epidemiological modelling of measles is discussed.

PKC-dependent stimulation of exocytosis by sulfonylureas in pancreatic beta cells

Hypoglycemic sulfonylureas represent a group of clinically useful antidiabetic compounds that stimulate insulin secretion from pancreatic beta cells. The molecular mechanisms involved are not fully understood but are believed to involve inhibition of potassium channels sensitive to adenosine triphosphate (KATP channels) in the beta cell membrane, causing membrane depolarization, calcium influx, and activation of the secretory machinery. In addition to these effects, sulfonylureas also promoted exocytosis by direct interaction with the secretory machinery not involving closure of the plasma membrane KATP channels. This effect was dependent on protein kinase C (PKC) and was observed at therapeutic concentrations of sulfonylureas, which suggests that it contributes to their hypoglycemic action in diabetics.

A novel regulatory mechanism for trimeric GTP-binding proteins in the membrane and secretory granule fractions of human and rodent beta cells

Recently we described roles for heterotrimeric and low-molecular-mass GTP-binding proteins in insulin release from normal rat islets. During these studies, we observed that a protein with an apparent molecular mass (37 kDa) similar to that of the beta subunit of trimeric GTP-binding proteins underwent phosphorylation in each of five classes of insulin-secreting cells. Incubation of the beta cell total membrane fraction or the isolated secretory granule fraction (but not the cytosolic fraction) with [gamma-32P]ATP or [gamma-32P]GTP resulted in the phosphorylation of this protein, which was selectively immunoprecipitated by an anti-serum directed against the common beta subunit of trimeric G-proteins. Disruption of the alpha beta gamma trimer (by pretreatment with either fluoroaluminate or guanosine 5'(-)[gamma-thio]triphosphate) prevented beta subunit phosphorylation. Based on differential sensitivities to pH, heat and the histidine-selective reagent diethyl pyrocarbonate (and reversal of the latter by hydroxylamine), the phosphorylated amino acid was presumptively identified as histidine. Incubation of pure beta subunit alone or in combination with the exogenous purified alpha subunit of transducin did not result in the phosphorylation of the beta subunit, but addition of the islet cell membrane fraction did support this event, suggesting that membrane localization (or a membrane-associated factor) is required for beta subunit phosphorylation. Incubation of phosphorylated beta subunit with G alpha.GDP accelerated the dephosphorylation of the beta subunit, accompanied by the formation of G alpha-GTP. Immunoblotting detected multiple alpha subunits (of Gi, G(o) and Gq) and at least one beta subunit in the secretory granule fraction of normal rat islets and insulinoma cells. These data describe a potential alternative mechanism for the activation of GTP-binding proteins in beta cells which contrasts with the classical receptor-agonist mechanism: G beta undergoes transient phosphorylation at a histidine residue by a GTP-specific protein kinase; this phosphate, in turn, may be transferred via a classical Ping-Pong mechanism to G alpha.GDP (inactive), yielding the active configuration G alpha.GTP in secretory granules (a strategic location to modulate exocytosis).

Specific co-ordinated regulation of PC3 and PC2 gene expression with that of preproinsulin in insulin-producing beta TC3 cells

Short-term (less than 2 h) glucose stimulation of isolated pancreatic islets specifically increases the biosynthesis of proinsulin and its converting enzymes PC2 and PC3 at the translation level. To determine whether gene expression of PC2 and PC3 was also regulated by longer-term (more than 6 h) glucose stimulation along with that of preproinsulin, studies were performed with the beta TC3 insulin-producing cell line. By Northern blot analysis, glucose maintained PC2 and PC3 mRNA levels in parallel with those of preproinsulin. After 48 h, mRNA levels of preproinsulin, PC2 and PC3 were, respectively, 2.9 (P < 0.05), 3.0 (P < 0.005) and 5.3 (P < 0.001) times greater in the presence of glucose than in beta TC3 cells cultured in the absence of glucose. Glucose-regulated PC2 and PC3 gene expression, like that of preproinsulin, was maximal at glucose concentrations above 5.5 mM. Studies of mRNA stability showed that the half-lives of PC2 (9 h) and PC3 (5 h) mRNA were much shorter than that of preproinsulin mRNA (over 24 h), but little effect of glucose on stability of these mRNAs was observed. Nuclear run-off analysis indicated that transcription of preproinsulin, PC2 and PC3 was modestly induced after 1 h exposure to 16.7 mM glucose. Therefore preproinsulin, PC2 and PC3 mRNA levels in beta TC3 cells were most probably maintained at the level of gene transcription. In contrast, elevation of cyclic AMP by forskolin had no effect on mRNA levels or gene transcription of preproinsulin, PC2 and PC3, despite a cyclic-AMP-induced phosphorylation of the cyclic AMP response element binding protein that correlated with a marked increase in cJun and cFos gene transcription in the same beta-cells. These results suggest that preproinsulin, PC2 and PC3 gene transcription can be specifically glucose-regulated in a mechanism that is unlikely to involve a key role for cyclic AMP. The co-ordinate increase in PC2 and PC3 mRNA levels with that of preproinsulin mRNA in response to chronic glucose represents a long-term means of catering for an increased demand on proinsulin conversion.

Glucose-regulated translational control of proinsulin biosynthesis with that of the proinsulin endopeptidases PC2 and PC3 in the insulin-producing MIN6 cell line

In the short term (< 2 h), proinsulin biosynthesis is predominately glucose regulated at the translational level; however, the details at the molecular level behind this mechanism are not well defined. One of the major hindrances for gaining a better understanding of the proinsulin biosynthetic mechanism has been a lack of an abundant source of beta-cells that express a phenotype of regulated proinsulin biosynthesis in the appropriate 2.8-16.7 mmol/l glucose range as defined in normal pancreatic islets. In this study, we demonstrate that in the MIN6 cell line, specific glucose-regulated translational control of proinsulin biosynthesis is present in the appropriate glucose concentration range. In addition to that of proinsulin, the biosynthesis of the two proinsulin conversion endopeptidases, PC2 and PC3, was coordinately glucose regulated in MIN6 cells, whereas that of the exopeptidase, carboxypeptidase H, was unaffected by glucose. Proinsulin, PC2 and PC3 biosynthesis was specifically stimulated over that of total MIN6 cell protein synthesis above a threshold of 4 mmol/l glucose that reached a maximum rate between 8 and 10 mmol/l glucose. Glucose-induced proinsulin, PC2, and PC3 biosynthesis was rapid (occurring after a 20-min lag period but reaching a maximum by 60 min), unaffected by the presence of actinomycin D; and in parallel experiments, stimulatory glucose concentrations did not alter MIN6 cell total preproinsulin, PC2, or PC3 mRNA levels. Thus, short-term (< 2 h) glucose stimulation of proinsulin, PC2 and PC3 biosynthesis in MIN6 cells, like that in isolated islets, was mediated at the translational level. Intracellular signals for mediating glucose-stimulated proinsulin PC2 and PC3 biosynthesis translation in MIN6 cells also appeared to be similar to those in pancreatic islets, requiring glucose metabolism and a supporting role for protein kinase A. However, protein kinase C or a Ca(2+)-dependent protein kinase did not appear to be required for glucose-regulated proinsulin biosynthesis in MIN6 cells, as in islets. MIN6 cells are the first beta-cell line that indicate glucose-regulated proinsulin biosynthesis translation essentially identical to that in differentiated islet beta-cells and will be an important experimental model to better define the mechanism of proinsulin biosynthesis in detail.

Increased secretory demand rather than a defect in the proinsulin conversion mechanism causes hyperproinsulinemia in a glucose-infusion rat model of non-insulin-dependent diabetes mellitus

Hyperproinsulinemia in non-insulin-dependent diabetes mellitus (NIDDM) is due to an increased release of proinsulin from pancreatic beta cells. This could reside in increased secretory demand placed on the beta cell by hyperglycemia or in the proinsulin conversion mechanism. In this study, biosynthesis of the proinsulin conversion enzymes (PC2, PC3, and carboxypeptidase-H [CP-H]) and proinsulin, were examined in islets isolated from 48-h infused rats with 50% (wt/vol) glucose (hyperglycemic, hyperinsulinemic, and increased pancreatic proinsulin to insulin ratio), 20% (wt/vol) glucose (normoglycemic but hyperinsulinemic), and 0.45% (wt/vol) saline (controls). A decrease in the islet content of PC2, PC3, and CP-H from hyperglycemic rats was observed. This reduction did not correlate with any deficiency in mRNA levels or biosynthesis of PC2, PC3, CP-H, or proinsulin. Furthermore, proinsulin conversion rate was comparable in islets from hyperglycemic and control rats. However, in islets from hyperglycemic rats an abnormal increased proportion of proinsulin was secreted, that was accompanied by an augmented release of PC2, PC3 and CP-H. Stimulation of the beta cell's secretory pathway by hyperglycemia, resulted in proinsulin being prematurely secreted from islets before its conversion could be completed. Thus, hyperproinsulinemia induced by chronic hyperglycemia likely results from increased beta cell secretory demand, rather than a defect in the proinsulin processing enzymes per se.

Rab3A effector domain peptides induce insulin exocytosis via a specific interaction with a cytosolic protein doublet

A key protein involved in the regulated exocytotic mechanism in neuroendocrine cells is the GTP-binding protein, Rab3A. Rab3A is thought to mediate exocytosis by an interaction of its effector domain with a putative effector protein. We demonstrate here that Rab3A effector domain peptides specifically stimulated insulin exocytosis in electroporated insulin-secreting cells (K0.5 activation, 6-8 microM) in a Ca(2+)-independent manner, although in the presence of Ca2+ insulin exocytosis was further potentiated. By using a 125I-radiolabeled photoactivated cross-linking Rab3A effector domain peptide, we identified a cytosolic protein doublet (REEP-1 and REEP-2), which specifically interacted with the Rab3A effector domain. Competitive inhibition studies revealed this protein-protein interaction to be at a concentration equivalent to that required for Rab3A effector domain peptides to trigger insulin exocytosis (Ki, 6-8 microM). Furthermore, under basal secretory conditions REEP-1 and -2 were membrane-associated, but upon stimulation of exocytosis they were released into a cytosolic fraction. Our results suggest that REEP-1 and -2 are part of the regulated exocytotic machinery, and their dissociation upon stimulation of hormone release (likely from a protein complex) may be essential to the mechanism that triggers regulated exocytosis in pancreatic beta-cells.

A Kex2-related endopeptidase activity present in rat liver specifically processes the insulin proreceptor

The insulin proreceptor is cleaved by limited proteolysis post-translationally at an Arg-Lys-Arg-Arg site to generate its mature alpha- and beta-subunit form. An 35S-labelled insulin proreceptor substrate preparation and a 15-mer peptide substrate that mimics the amino acid sequence around and including the insulin proreceptor processing site (IRP-peptide) has revealed an endopeptidase activity that catalyses insulin proreceptor cleavage in a rat liver subcellular fraction. Under optimal conditions, normal 35S-labelled insulin proreceptor substrate processing by this fraction was quantitative. This fraction was not able to process an 35S-labelled insulin proreceptor variant substrate (where the Arg-1 of the tetrabasic cleavage site had been replaced by Ala-1), similarly to previous in vivo observations, suggesting that this endopeptidase activity has physiological relevance. Biochemical characterization of the insulin proreceptor/IRP-peptide processing revealed this rat liver endopeptidase activity to have a broad pH range (> 70% maximal activity between pH 5.5 and 10.0) and a pH optimum of pH 8-10. It was Ca(2+)-dependent activity, maximally active between 0.5 and 5 mM Ca2+ and half-maximally activated between 50 and 90 microM Ca2+. Endoproteolytic activity was not inhibited by group-specific inhibitors of serine-, cysteinyl or aspartyl proteinases or by 1,10-phenanthroline; however, EDTA and 1,2-cyclohexanediaminetetraacetic acid did inhibit the activity, but this was accounted for by Ca2+ chelation. The IRP-peptide substrate assay enabled measurement of an apparent Km of 22 microM and a Vmax of 18.6 pmol/min for this endopeptidase activity. These biochemical characteristics suggest that insulin proreceptor processing endopeptidase activity to be a legitimate member of the Kex2-related proprotein convertase family. Immunoblotting detected furin and PACE4 proteins (both members of this family) to be present in the rat liver subcellular fraction containing insulin proreceptor processing activity. Since the biochemical characteristics of the insulin proreceptor processing endopeptidase activity mostly resembled those of furin activity, it is likely that insulin proreceptor proteolytic maturation can be catalysed by furin in the liver.

What beta-cell defect could lead to hyperproinsulinemia in NIDDM? Some clues from recent advances made in understanding the proinsulin-processing mechanism

Pancreatic beta-cell dysfunction is a characteristic of non-insulin-dependent diabetes mellitus (NIDDM). An aspect of this dysfunction is that an increased proportion of proinsulin is secreted, but an actual beta-cell defect that leads to hyperproinsulinemia is unknown. Nevertheless, an impairment in beta-cell proinsulin conversion mechanism has been suggested as the most likely cause. Insulin is produced from its precursor molecule, proinsulin, by limited proteolytic cleavage at two dibasic sequences (Arg31, Arg32 and Lys64, Arg65). Two endopeptidase activities catalyze this cleavage: PC2 and PC3. PC2 endopeptidase cleaves predominately at Lys64, Arg65, and PC3 endopeptidase cleaves at Arg31, Arg32. The recent identification and characterization of these endopeptidases has enabled a better understanding of the human proinsulin-processing mechanism. In particular, experimental evidence suggests that the majority of human proinsulin processing is sequential. PC3 cleaves proinsulin first to generate a proinsulin conversion intermediate that is the preferred substrate of PC2. Both PC2 and PC3 activities are influenced by Ca2+ and pH, but the more stringent Ca2+ and pH requirements of PC3 suggest it as the most likely enzyme to regulate proinsulin conversion, as well as initiate it. When an increased demand is placed on the proinsulin-processing mechanism by a glucose-stimulated increase in proinsulin biosynthesis, there is a coordinate increase in PC3 biosynthesis (but not in PC2). This supports PC3 as the key endopeptidase that regulates proinsulin processing. In this perspective, the current concepts of the enzymology and regulation of proinsulin conversion at a molecular level are reviewed.(ABSTRACT TRUNCATED AT 250 WORDS)

PACE4 is a member of the mammalian propeptidase family that has overlapping but not identical substrate specificity to PACE

Proteins that transit the constitutive pathway of secretion frequently require proteolytic processing after a pair of basic amino acids to attain their full functional activity. A ubiquitously expressed calcium-dependent subtilisin-like serine protease, named PACE or furin, can cleave precursor polypeptides specifically at pairs of basic amino acids where an arginine residue is present in the P4 position. Another member of this protease family, PACE4, was cloned recently by a PCR-based strategy and was also shown to be ubiquitously expressed. We have expressed PACE4 by transient DNA transfection of COS-1 cells and have shown that the cDNA encodes a 120-kDa polypeptide that is present in cell extracts but not in conditioned medium of transfected cells. The substrate specificities of PACE and PACE4 for cleavage of pro-von Willebrand factor were studied in parallel using a transient DNA cotransfection system. Like PACE, PACE4 was able to process pro-vWF to its mature form, and efficient cleavage required both the P4 arginine and the P2 lysine. These data, taken together with previously published data showing that PACE4 cannot process pro-factor IX, demonstrate that PACE and PACE4 have overlapping but not identical substrate specificities. Further differences between PACE and PACE4 specificities were elucidated by monitoring inhibition of processing activity mediated by the serine protease inhibitor alpha 1-antitrypsin Pittsburgh mutant. Pro-vWF processing by PACE was inhibited by expression of the alpha 1-antitrypsin Pittsburgh mutant, whereas processing of pro-vWF by PACE4 was not affected.(ABSTRACT TRUNCATED AT 250 WORDS)

The biosynthesis of the subtilisin-related proprotein convertase PC3, but no that of the PC2 convertase, is regulated by glucose in parallel to proinsulin biosynthesis in rat pancreatic islets

The biosynthesis of proinsulin is specifically stimulated by glucose in the pancreatic beta-cell, and this, in turn, places an increased demand on the mechanism for proinsulin to insulin conversion. Proteolytic proinsulin processing is catalyzed by two endopeptidases putatively identified as the subtilisin-related PC2 and PC3 convertases (Bennett, D. L., Bailyes, E. M., Nielson, E., Guest, P. C., Rutherford, N. G., Arden, S. D., and Hutton, J. C. (1992) J. Biol. Chem. 267, 15229-15236; Bailyes, E. M., Shennan, K. I. J., Seal, A. J., Smeekens, S. P., Steiner, D. F., Hutton, J. C., and Docherty, K. (1992) Biochem. J. 285, 391-394). In this study, we demonstrate in isolated rat pancreatic islets that the biosynthesis of PC3 was specifically stimulated by glucose relatively parallel to that of proinsulin. In contrast, however, PC2 biosynthesis was not glucose-regulated. The stimulation of PC3 and proinsulin biosynthesis was observed above a threshold of 4 mM glucose and reached a maximum (about 7-10-fold) above 10 mM glucose concentrations. Glucose stimulation for PC3 and proinsulin biosynthesis was rapid (occurring within 20 min and reaching a maximum by 60 min) and was not affected by the additional presence of actinomycin D, suggesting regulation predominantly at the translational level. Moreover, the intracellular signals for glucose-stimulated PC3 and proinsulin biosynthesis appeared to be similar, requiring the metabolism of glucose. PC3 has been implicated as the key endopeptidase in proinsulin to insulin conversion, in that it is the enzyme which preferentially initiates the process (Rhodes, C. J., Lincoln, B., and Shoelson, S. E. (1992) J. Biol. Chem. 267, 22719-22727). We suggest that co-ordinate stimulation of PC3 biosynthesis, along with that of its proinsulin substrate, elucidates an additional control point by which the mechanism of proprotein processing might be regulated.

Processing of proopiomelanocortin by insulin secretory granule proinsulin processing endopeptidases

A lysed preparation of isolated insulin secretory granules efficiently cleaved murine proopiomelanocortin (mPOMC) at physiologically important Lys-Arg processing sites. This processing was mostly attributed to an activity that co-eluted with the proinsulin processing type-II endopeptidase from anion exchange chromatography (Lys-Arg-directed; Davidson, H. W., Rhodes, C. J., and Hutton, J. C. (1988) Nature 333, 93-96). The principal peptide hormone products generated by the insulin secretory granule lysate were identified by specific radioimmunoassay and NH2-terminal microsequencing analysis of high performance liquid chromatography-separated products as alpha-melanocyte-stimulating hormone, corticotropin-like intermediate, gamma-lipotropin, beta-endorphin-(1-31), 18-kDa NH2-terminal fragment and, to a lesser extent, adrenocorticotrophin and beta-lipotropin. This processing had an acidic pH optimum (pH 5-5.5) and was Ca(2+)-dependent (K0.5 activation = 5-80 microM). With increasing Ca2+ concentrations there was an increase in the extent to which mPOMC was processed. The in vitro processing of mPOMC by the insulin secretory granule endopeptidase activity reported here is in excellent agreement with the in vivo processing of this prohormone by a combination of PC2 and PC3, candidates of prohormone endpeptidase, in gene transfer studies with cells that express the regulated secretory pathway (Thomas, L., Leduc, R., Thorne, B. A., Smeekens, S. S., Steiner, D. F., and Thomas, G. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 5297-5301).

Preferential cleavage of des-31,32-proinsulin over intact proinsulin by the insulin secretory granule type II endopeptidase. Implication of a favored route for prohormone processing

Two Ca(2+)-dependent endopeptidase activities are involved in proinsulin to insulin conversion: type I cleaves COOH-terminal to proinsulin Arg31-Arg32 (B-chain/C-peptide junction); and type II preferentially cleaves at the Lys64-Arg65 site (C-peptide/A-chain junction). To further understand the mechanism of proinsulin processing, we have investigated types I and II endopeptidase processing of intact proinsulin in parallel to that of the conversion intermediates, des-31,32-proinsulin and des-64,65-proinsulin. The type I processed des-64,65-proinsulin and proinsulin at the same rate. In contrast, the type II endopeptidase processed des-31,32-proinsulin at a much faster rate (> 19-fold; p < 0.001) than it did intact proinsulin. Furthermore, unlabeled proinsulin concentrations required for competitive inhibition of 125I-labeled des-64,65-proinsulin and 125I-proinsulin processing by a purified insulin secretory granule lysate were similar (ID50 = 14-16 microM), whereas inhibition of 125I-labeled des-31,32-proinsulin processing required a higher nonradiolabeled proinsulin concentration (ID50 = 197 microM). Synthetic peptides corresponding to the sequences surrounding Lys64-Arg65 (AC-peptide/substrate) and Arg31-Arg32 (BC-peptide/substrate) of human proinsulin were synthesized for use as specific substrates or competitive inhibitors. Cleavage of the BC-substrate by type I and AC-substrate by type II was COOH-terminal of the dibasic sequence, with similar Ca(2+)-and pH requirements previously observed for proinsulin cleavage. Apparent Km and Vmax for type I processing of the BC-substrate was Km = 20 microM; Vmax = 22.8 pmol/min, and for type II processing of the AC-substrate was Km = 68 microM; Vmax = 97 pmol/min. In competitive inhibition assays, the BC-peptide similarly blocked insulin secretory granule lysate processing of des-64,65-proinsulin and proinsulin (ID50 = 45-55 microM), but did not inhibit des-31,32-proinsulin processing. However, the AC-peptide preferentially inhibited insulin secretory granule lysate processing of des-31,32-proinsulin (ID50 = microM) compared to proinsulin (ID50 = 330 microM), and not des-64,65-proinsulin. We conclude that the type I endopeptidase recognized des-64,65-proinsulin and proinsulin as similar substrates, whereas the type II endopeptidase has a stronger preference for des-31,32-proinsulin compared to intact proinsulin. Furthermore, we suggest that in intact proinsulin there exists a constraint to efficient processing that is relieved following type I processing. Structural flexibility, in addition to the presence of Lys64-Arg65, therefore appears to be important for type II endopeptidase specificity and may provide a molecular basis for a preferential route of proinsulin conversion via des-31,32-proinsulin.

Fluorometric assay of a calcium-dependent, paired-basic processing endopeptidase present in insulinoma granules

A novel fluorogenic substrate Cbz-Arg-Ser-Lys-Arg-AMC (RSKR-AMC) was used to characterize Ca(++)-activated proteolytic activity present in purified insulinoma secretory granules. Secretory granules efficiently cleaved this substrate in a time- and protein-dependent manner; the hydrolysis rate was between 2 and 4 pmol/min/ug of protein, with an apparent Km of 55 microM. Greater than 90% of the activity against this substrate was dependent on the presence of Ca++, with half-maximal stimulation obtained at 100 microM Ca++. The pH optimum of enzymatic activity was 5.5-6, and the profile of inhibition by various proteinase inhibitors was similar to that previously described for the type I and II proinsulin processing enzymes. These biochemical characteristics and co-elution of the RSKR-AMC processing activity with the type II endopeptidase activity on anion-exchange chromatography suggest that the new assay selectively detects the Lys-Arg-directed, or type II, proinsulin processing endopeptidase. This fluorogenic assay is more quantitative, sensitive and rapid than methods previously used, and therefore presents a significant improvement for the study of similar Ca(++)-activated processing endopeptidases.

Co-secretion of carboxypeptidase H and insulin from isolated rat islets of Langerhans

The release of carboxypeptidase H activity from isolated rat islets was determined and compared to the secretion of immunoreactive insulin. Analysis of pancreatic islet cells sorted into beta and non-beta types indicated that approx. 80% of islet carboxypeptidase H activity is present in the beta cell. The release of both insulin and carboxypeptidase H was stimulated markedly by increasing the glucose concentration in the medium from 2.8 to 28 mM. The fractional release was in accordance with the observed cellular distribution of both proteins. The secretory response was biphasic with time, with an initial rapid transient phase of release within 5 min, followed by a more sustained response. The concentration-dependencies of glucose stimulation of release of insulin and carboxypeptidase H were similar, with a threshold for stimulation around 5.6 mM-glucose and maximal stimulatory response at 16.7-28 mM-glucose. The release of both proteins was inhibited by 20 mM-mannoheptulose, removal of Ca2+ from the medium and addition of 1 microM-noradrenaline. The combination of 10 mM-4-methyl-2-oxopentanoate and 10 mM-glutamine stimulated the release of carboxypeptidase H and insulin, as did 3-isobutyl-1-methylxanthine and 350 microM-tolbutamide in the presence of glucose. It is evident that carboxypeptidase H is released from the pancreatic beta-cell by an exocytotic process from the same intracellular compartment as insulin. The release of carboxypeptidase H by a constitutive process was at best equivalent to 0.4%/h, or less than 2% of the maximal rate of release via the regulated pathway. It is concluded that carboxypeptidase H can be used as a sensitive index of beta-cell secretion and an alternative marker to the insulin-related peptides.

Proinsulin endopeptidase substrate specificities defined by site-directed mutagenesis of proinsulin

Two endopeptidases are involved in the conversion of proinsulin; a type I activity directed at the B chain, Arg31,Arg32, C-peptide junction, and type II which cleaves the C-peptide, Lys64,Arg65, A chain junction. To define further the substrate specificities of these enzymes, a series of mutant preproinsulin cDNAs were generated by site-directed and deletion mutagenesis. These were inserted into pT7 plasmids and capped cRNA transcripts synthesized, that were then microinjected into Xenopus oocytes. Oocytes were biosynthetically radiolabeled with [3H]leucine and the secreted peptides (greater than 95% present as unprocessed proinsulins) then incubated with types I and II endopeptidase activities prepared from isolated insulinoma secretory granules. The reaction products were analyzed by high performance liquid chromatography. Des-38-62-proinsulin, in which all but six amino acids of C-peptide were deleted was not processed by either enzyme. The mutant Lys64,Arg65 to Thr64,Arg65 was not cleaved by the type II enzyme but was still a substrate for the type I enzyme. The mutant Arg31,Arg32 to Arg31,Gly32 correspondingly was not cleaved by the type I enzyme; however, in this case it was not attacked by the type II enzyme. These results indicate that not only is the presence of a dibasic sequence essential, but also that the secondary structure of the protein is important in determining whether the prohormone is susceptible to proteolytic processing.

Proalbumin to albumin conversion by a proinsulin processing endopeptidase of insulin secretory granules

A lysate of purified insulin secretory granules, which contains two types of proinsulin processing activity (type 1, Arg-Arg-directed and type II, Lys-Arg-directed (Davidson, H.W., Rhodes, C.J., and Hutton, J. C. (1988) Nature 333, 93-96), was found to process proalbumin by specific proteolytic cleavage of the COOH-terminal side of the Arg-2-Arg-1 sequence. The subcellular distribution of proalbumin processing activity in insulinoma tissue paralleled that for proinsulin conversion and occurred principally in a secretory granule fraction. Cleavage appeared to result from the Arg-Arg-directed type 1 proinsulin processing endo-peptidase. It was Ca2+-dependent (K0.5 activation = 1.0-1.5 mM Ca2+), unaffected by group-specific inhibitors of serine, cysteinyl, or aspartyl proteinases, and had an acidic pH optimum (5.5). Active-site inhibitor studies showed this activity had a preference for dibasic over monobasic amino acid sequences and indicated that the sequence of the dibasic site was an important determinant of the susceptibility of the substrate to cleavage. The activity did not process the proalbumin Christchurch mutant (Arg-2-Arg-1 to Arg-2-Gln-1). It was inhibited by the variant alpha 1-antitrypsin Pittsburgh (Met358 to Arg358; K0.5 = 100 nM) but not by other related proteins normally co-secreted with albumin from hepatocytes, namely alpha 1-antitrypsin M, alpha 2-macroglobulin, or antithrombin III. The insulin secretory granule proalbumin processing activity was indistinguishable from a proalbumin endopeptidase reported in rat liver membranes and similar to the yeast KEX-2 protease. These findings suggest that a highly conserved set of proprotein endopeptidases exists, which are specific for a dibasic sequence but broadly specific for proprotein substrates. Such enzymic activities appear to be active within both the constitutive and regulated pathways of secretion. Intraorganellar Ca2+ and pH appear to play a key role in regulating their activities.

The inhibition of proinsulin-processing endopeptidase activities by active-site-directed peptides

Inhibitor studies were performed on the two endopeptidase activities involved in proinsulin conversion in isolated insulin secretory granules [Davidson, Rhodes & Hutton (1988) Nature (London) 333, 93-96]. The active-site-directed peptides L-alanyl-L-arginyl-L-arginylmethyldimethylsulphonium and L-alanyl-L-lysyl-L-arginylmethyldimethylsulphonium inhibited these activities in accordance with the observed cleavage pattern, suggesting that the primary amino acid sequence of the dibasic site was an important determinant of the endopeptidase substrate specificities.

Regulation of the biosynthesis of insulin-secretory-granule proteins. Co-ordinate translational control is exerted on some, but not all, granule matrix constituents

The regulation of the biosynthesis of the insulin-secretory-granule matrix proteins insulin II, chromogranin A and carboxypeptidase H was studied in isolated rat islets of Langerhans. Islets were labelled with [35S]-methionine, and incorporation into total protein was determined by trichloroacetic acid precipitation and that into specific proteins by immunoprecipitation followed by polyacrylamide-gel electrophoresis and fluorography. Islets incubated in the presence of 16.7 mM-glucose incorporated 3 times as much [35S]-methionine into total protein as did islets incubated with 2.8 mM-glucose. The same conditions produced more than a 20-fold increase in incorporation into both proinsulin and chromogranin A, with no observable effect on carboxypeptidase H. The concentration-dependencies of the glucose-stimulated synthesis of chromogranin A and proinsulin were parallel, and in both cases the response to 16.7 mM-glucose was typified by an initial lag of 20 min, followed by a rapid activation to a new steady state over the ensuing 40 min. Synthesis of total protein, although activated to a lesser extent, responded with similar kinetics. Extracellular Ca2+ depletion did not affect the basal or glucose-stimulated biosynthesis of any of the proteins under investigation. Mannoheptulose (20 mM) abolished glucose-stimulated synthesis of insulin, chromogranin A and total protein, but had no effect on the synthesis of carboxypeptidase H. It is concluded that the biosynthesis of insulin and chromogranin A is regulated principally at the translational level by the same intracellular signal generated from the metabolism of glucose. Such regulation is not common to all insulin-secretory-granule proteins, since the synthesis of carboxypeptidase H was unaffected by the same stimulus.

Intraorganellar calcium and pH control proinsulin cleavage in the pancreatic beta cell via two distinct site-specific endopeptidases

Insulin is produced from an inactive precursor, proinsulin, through initial endoproteolytic cleavage at sites marked by pairs of basic amino-acid residues. We report here that lysates of insulin secretory granules contain two distinct Ca-dependent acidic endoproteases; one (type I) cleaving exclusively on the C-terminal side of Arg 31.Arg 32 (B-chain/C-peptide junction), the other (type II) preferentially on the C-terminal side of Lys 64.Arg 65 of proinsulin (C-peptide/A-chain junction). The Ca and pH requirements of these proteinases suggested that the type-II proteinase would be active in the Golgi apparatus and the secretory granule, whereas type-I activity would be compatible only with the intragranular environment. Kinetic analyses of (pro)insulin conversion intermediates in [35S]methionine-pulsed rat islets support this supposition. Our results suggest a simple mechanism whereby different dibasic sites can be cleaved in different cellular compartments. In conjunction with the regulation of the ionic composition of such compartments and the operation of post-Golgi segregation, our results also suggest how proteolytic conversion of diverse proproteins destined for different cellular sites can occur differentially and in a regulated manner.

The intracellular handling of insulin-related peptides in isolated pancreatic islets. Evidence for differential rates of degradation of insulin and C-peptide

Islets of Langerhans isolated from adult rats were maintained in tissue culture for 3 days in the continued presence of [3H]leucine. Labelled proinsulin, C-peptide and insulin were measured by quantitative h.p.l.c., a method which also allowed for resolution of C-peptide I and II, and of insulin I and II (the products of the two rat insulin genes). The results showed that: (1) at early times, proinsulin was the major radiolabelled product; with progressive time in culture, intra-islet levels of [3H]proinsulin decreased, despite continuous labelling with [3H]leucine, indicating that the combined rates of proinsulin conversion into insulin and of proinsulin release, exceeded the rate of synthesis; (2) insulin I levels were always greater than those of insulin II, both in the islets and for products released to the medium; (3) the molar ratio of [3H]insulin I and II to their respective 3H-labelled C-peptides increased with time for products retained within islets, reaching a value close to 3:1 by 3 days; by contrast, for products released to the medium during the culture period, the ratio was always close to unity; (4) when islets were incubated with [3H]leucine for 2 days, and then left for a further 1 day without label (chase period), the intra-islet [3H]insulin/[3H]C-peptide ratios rose to values as high as 9:1. Again, for material released to the medium, the values were close to 1:1; (5) it is concluded that C-peptide is degraded more rapidly than insulin within islet cells, thereby accounting for the elevated insulin/C-peptide ratios. The difference between the ratios observed in the islets and those for material released to the medium is taken to indicate that degradation occurs in a discrete cellular compartment and not in the secretory granule itself.

Stimulation by ATP of proinsulin to insulin conversion in isolated rat pancreatic islet secretory granules. Association with the ATP-dependent proton pump

Isolated rat pancreatic islets were pulse-labeled for 5 min with [3H]leucine then chased for 25 min, during which time endogenously labeled [3H]proinsulin becomes predominantly compartmented in immature secretory granules. The islets were then homogenized in isotonic sucrose (pH 7.4) and a beta-granule preparation obtained by differential centrifugation and discontinuous sucrose gradient ultracentrifugation. This preparation was enriched 8-fold in beta-granules. Aside from contamination with mitochondria and a limited number of lysosomes, the beta-granule preparation was essentially free of any other organelles involved in proinsulin synthesis and packaging (i.e. microsomal elements and, more particularly, Golgi complex). Conversion of endogenously labeled [3H]proinsulin was followed in this beta-granule fraction for up to 2 h at 37 degrees C in a buffer (pH 7.3) that mimicked the cationic constituents of B-cell cytosol, during which time 92% of the beta-granules remained intact. Proinsulin conversion was analyzed by high performance liquid chromatography. The rate of proinsulin conversion to insulin was stimulated by 2.2 +/- 0.1-fold (n = 6) (at a 60-min incubation) in the presence of ATP (2 mM) and an ATP regenerating system compared to beta-granule preparations incubated without ATP. This ATP stimulation was abolished in the presence of beta-granule proton pump ATPase inhibitors (tributyltin, 2.5 microM, or 1,3-dicyclohexylcarbodiimide, 50 microM). Inhibitors of mitochondrial proton pump ATPases (sodium azide, 20 mM, or oligomycin, 10 micrograms/ml) had no effect on the ATP stimulation of proinsulin conversion. When granules were incubated in a more acidic buffer (pH 5.5), proinsulin conversion was increased relative to that at pH 7.3. At pH 5.5, ATP no longer stimulated conversion, and tributyltin and 1,3-dicyclohexylcarbodiimide had no effect. Disrupted granules only converted proinsulin to a limited extent, and neither ATP nor the inhibitors affected conversion. It is therefore suggested that ATP stimulation of proinsulin conversion in isolated, intact, beta-granules is secondary to intragranular acidification by an ATP-dependent proton pump (reflecting the low pH optimum for proinsulin conversion), rather than ATP dependence of converting activity per se.

Newly synthesized proinsulin/insulin and stored insulin are released from pancreatic B cells predominantly via a regulated, rather than a constitutive, pathway

The pancreatic B cell has been used as a model to compare the release of newly synthesized prohormone/hormone with that of stored hormone. Secretion of newly synthesized proinsulin/insulin (labeled with [3H]leucine during a 5-min pulse) and stored total immunoreactive insulin was monitored from isolated rat pancreatic islets at basal and stimulatory glucose concentrations over 180 min. By 180 min, 15% of the islet content of stored insulin was released at 16.7 mM glucose compared with 2% at 2.8 mM glucose. After a 30-min lag period, release of newly synthesized (labeled) proinsulin and insulin was detected; from 60 min onwards this release was stimulated up to 11-fold by 16.7 mM glucose. At 180 min, 60% of the initial islet content of labeled proinsulin was released at 16.7 mM glucose and 6% at 2.8 mM glucose. Specific radioactivity of the released newly synthesized hormone relative to that of material in islets indicated its preferential release. A similar degree of isotopic enrichment of released, labeled products was observed at both glucose concentrations. Quantitative HPLC analysis of labeled products indicated that glucose had no effect on intracellular proinsulin to insulin conversion; release of both newly synthesized proinsulin and insulin was sensitive to glucose stimulation; 90% of the newly synthesized hormone was released as insulin; and only 0.5% of proinsulin was rapidly released (between 30 and 60 min) in a glucose-independent fashion. It is thus concluded that the major portion of released hormone, whether old or new, processed or unprocessed, is directed through the regulated pathway, and therefore the small (less than 1%) amount released via a constitutive pathway cannot explain the preferential release of newly formed products from the B cell.

Glucose stimulates the biosynthesis of rat I and II insulin to an equal extent in isolated pancreatic islets

The effects of glucose on insulin biosynthesis were studied by measuring the incorporation of radiolabelled amino acids into proinsulin/insulin in isolated rat islets. The islets were pulse labelled for 15 min with [3H]leucine (present in rat insulin I and II) or [35S]methionine (unique to rat insulin II) and then incubated for a 165 min post-label (chase) period during which the majority of labelled proinsulin was converted to insulin but under conditions whereby greater than 95% of radiolabelled proinsulin or insulin was retained in the islets. The newly synthesized, labelled, insulin was analyzed by high performance liquid chromatography. Rat I and II insulin biosynthesis was stimulated by 16.7 mM glucose to the same extent.

High-performance liquid chromatography (HPLC): a rapid, flexible and sensitive method for separating islet proinsulin and insulin

Evaluating islet function in vitro involves studying both insulin biosynthesis and release. For the former, it is necessary to resolve insulin from its precursor, proinsulin. This has been achieved in the past by various procedures, each of which suffers from major drawbacks in terms of resolution and the time involved. We show here that reversed phase high-performance liquid chromatography (HPLC) outperforms previous methods for separating proinsulin from insulin in islet extracts without any prepurification or concentration steps. This HPLC method is rapid (90 min for a complete cycle, including washing the column) and reproducible, while allowing for unambiguous separation and quantification of proinsulin and insulin.

Effects of glucose and D-3-hydroxybutyrate on human pancreatic islet cell function

Beta-Cell function in human islets derived from a number of kidney donors was investigated by using various types of islet preparations. With fresh islets, both insulin release and biosynthesis were increased by raising glucose concentrations, although the response was a variable one. In fresh islets, the effects of 5 mmol of glucose/l on release were potentiated by 10 mmol of D-3-hydroxybutyrate/l. Insulin release at 20 mmol of glucose/l was inhibited by adrenaline (0.1 mmol/l), and potentiated by theophylline (10 mmol/l) in the presence of 5 mmol of glucose/l, in islets cultured for 4 days. After culture for 8 days, islets still showed an increase in insulin release and biosynthesis in response to glucose. Pancreas slices derived from fresh human tissue also responded to increasing concentrations of glucose with a sigmoidal curve for insulin release.

2-5A-dependent endoribonuclease activity and effects of A2'p5'A2'p5'A (2-5A core) in mouse pancreatic islets

Glucose-stimulated (pro)insulin biosynthesis is markedly inhibited is mouse pancreatic islets incubated with micromolar concentrations of 2-5A 'core', (A2'p5'A2'p5'A). Total protein synthesis was also reduced, but to a lesser extent, while insulin release and total insulin content of the islets was untouched by 2-5A 'core'. Evidence is given for the presence of a 2-5A-dependent endoribonuclease which mediates these effects.

Effect of human lymphoblastoid interferon on insulin synthesis and secretion in isolated human pancreatic islets

Human islets of Langerhans were isolated from the pancreas removed from a 13-year-old female transplant donor. The islets were incubated in a culture medium for 24 h in the presence of human lymphoblastoid interferon (1000 units/ml). Insulin secretion, proinsulin biosynthesis, total protein biosynthesis and total insulin content were assessed at various concentrations of glucose in the presence of interferon. In interferon-treated islets glucose-stimulated insulin secretion was unaltered from that of control islets; however, glucose-stimulated proinsulin biosynthesis was specifically inhibited by interferon (48%, p less than 0.025). Total protein biosynthesis and total insulin content were not significantly affected by interferon.

Long-term biochemical changes in the islets of Langerhans in mice following infection with encephalomyocarditis virus

A mild diabetes was induced in mice following the inoculation of a myocardial variant of encephalomyocarditis virus. The majority of infected mice developed a transient hyperglycaemia and only a few mice exhibited a chronic elevation of blood glucose. Infected mice were selected for study if their non-fasting blood glucose levels were above two standard deviations of values in control mice for at least 5 consecutive days. The pancreas from selected inoculated mice and control mice was removed 60 days after infection; random blood glucose levels at this time had returned to normal. Isolated islets of Langerhans from such previously 'diabetic' mice were used to determine the rate of insulin secretion, insulin biosynthesis and cyclic AMP accumulation in response to various glucose concentrations. At glucose (16 mmol/l), both insulin release and biosynthesis were depressed in previously infected animals. At glucose (2 and 10 mmol/l), while insulin biosynthesis was unchanged in infected animals, insulin release was increased. Cyclic AMP accumulation in response to glucose (20 mmol/l) was found to be significantly elevated in islets from infected animals and especially in response to glucose (20 mmol/l) in the presence of isobutyl methyl xanthine (1 mmol/l). Infected mice exhibited a reduction of total pancreatic insulin content in comparison with control mice. The insulin content of isolated islets of Langerhans from infected and control mice was found to be the same. Serum insulin levels, however, from infected mice were higher than in control sera.(ABSTRACT TRUNCATED AT 250 WORDS)

An evaluation of the use of Sep-Pak C18 cartridges for the extraction of vitamin D3 and some of its metabolites from plasma and urine

The use of Sep-Pak C18 cartridges for the extraction of vitamin D and some of its metabolites from plasma and urine has been evaluated by studying the recovery of added tritiated secosteroids. The preparation of the cartridges, recoveries, extraction and elution with a number of solvents, effect of varying flow rates for application and elution, and the effect of increasing volumes of plasma and urine have been investigated. Two methods for the application of secosteroids present in plasma to Sep-Pak C18 cartridges have been examined, using methyl cyanide extracts removing precipitated protein by centrifugation, and using acidified methanolic plasma. Methyl cyanide extracts applied to Sep-Pak C18 cartridges and eluted with methanol or methyl cyanide gave the cleanest extracts suitable for direct HPLC. Acidified methanolic plasma, applied to Sep-Pak C18 cartridges and eluted with methanol or methyl cyanide gave extracts which could not be applied directly to an HPLC--further fractionation using Sep-Pak SIL cartridges was necessary. Recoveries of added tritiated secosteroids using both methods were greater than 80% with the exception of vitamin D itself which was poorly recovered--methyl cyanide extraction giving only 30% recovery and use of acidified methanolic plasma giving 66% recovery.

Radiological Investigation of the Small Intestine by Small Bowel Enema Technique

This paper records some experience with the Scott Harden technique of small intestinal intubation for performing small bowel enemas. It is particularly recommended for searching for obstructive or infiltrative lesions of the small intestine.