Decreased transport of leptin across the blood-brain barrier in rats lacking the short form of the leptin receptor

Leptin is produced in adipose tissue in the periphery, but its satiety effect is exerted in the CNS that it reaches by a saturable transport system across the blood-brain barrier (BBB). The short form of the leptin receptor has been hypothesized to be the transporter, with impaired transport of leptin being implicated in obesity. In Koletsky rats, the splice variant that gives rise to the short form of the leptin receptor contains a point mutation that results in marked obesity. We studied the transport of leptin across the BBB in Koletsky rats and found it to be significantly less than in their lean littermates. By contrast, Sprague-Dawley rats matched in weight to each of these two groups showed no difference in the blood-to-brain influx of leptin. HPLC showed that most of the leptin crossing the BBB in rats remained intact and capillary depletion showed that most of the leptin reached the parenchyma of the brain. The results indicate that the short form of the leptin receptor is involved in the transport of leptin across the BBB.

Analysis of recombinant merozoite surface protein-1 of Plasmodium falciparum expressed in mammalian cells

Synthetic chimeric DNA constructs with a reduced A + T content coding for full-length merozoite surface protein-1 of Plasmodium falciparum (MSP1) and three fragments thereof were expressed in HeLa cells. To target the recombinant proteins to the surface of the host cell the DNA sequences coding for the N-terminal signal sequence and for the putative C-terminal recognition/attachment signal for the glycosyl-phosphatidyl-inositol (GPI)-anchor of MSP1 were replaced by the respective DNA sequences of the human decay-accelerating-factor (DAF). The full-length recombinant protein, hu-MSP1-DAF, was stably expressed and recognised by monoclonal antibodies that bind to the N-terminus or the C-terminus of the native protein, respectively. Its apparent molecular mass is higher as compared to the native protein and it is post-translationally modified by attachment of N-glycans whereas native MSP1 is not glycosylated. Immunofluorescence images of intact cells show a clear surface staining. After permeabilization hu-MSP1-DAF can be detected in the cytosol as well. As judged by protease treatment of intact cells 25% of recombinant MSP1 is located on the surface. This fraction of hu-MSP1-DAF can be cleaved off the cell membrane by phosphatidylinositol-specific phospholipase C indicating that the protein is indeed bound to the cell membrane via a GPI-anchor. Human erythrocytes do not adhere to the surface of mammalian cells expressing either of the constructs made in this study.

Linear regression for bivariate censored data via multiple imputation

Bivariate survival data arise, for example, in twin studies and studies of both eyes or ears of the same individual. Often it is of interest to regress the survival times on a set of predictors. In this paper we extend Wei and Tanner's multiple imputation approach for linear regression with univariate censored data to bivariate censored data. We formulate a class of censored bivariate linear regression methods by iterating between the following two steps: 1. the data is augmented by imputing survival times for censored observations; 2. a linear model is fit to the imputed complete data. We consider three different methods to implement these two steps. In particular, the marginal (independence) approach ignores the possible correlation between two survival times when estimating the regression coefficient. To improve the efficiency, we propose two methods that account for the correlation between the survival times. First, we improve the efficiency by using generalized least squares regression in step 2. Second, instead of generating data from an estimate of the marginal distribution we generate data from a bivariate log-spline density estimate in step 1. Through simulation studies we find that the performance of the two methods that take the dependence into account is close and that they are both more efficient than the marginal approach. The methods are applied to a data set from an otitis media clinical trial.

Peptides crossing the blood-brain barrier: some unusual observations

An interactive blood-brain barrier (BBB) helps regulate the passage of peptides from the periphery to the CNS and from the CNS to the periphery. Many peptides cross the BBB by simple diffusion, mainly explained by their lipophilicity and other physicochemical properties. Other peptides cross by saturable transport systems. The systems that transport peptides into or out of the CNS can be highly specific, transporting MIF-1 but not Tyr-MIF-1, PACAP38 but not PACAP27, IL-1 but not IL-2, and leptin but not the smaller ingestive peptides NPY, orexin A, orexin B, CART (55-102[Met(O)(67)]), MCH, or AgRP(83-132). Although the peptides EGF and TGF-alpha bind to the same receptor, only EGF enters by a rapid saturable transport system, suggesting that receptors and transporters can represent different proteins. Even the polypeptide NGF enters faster than its much smaller subunit beta-NGF. The saturable transport of some compounds can be upregulated, like TNF-alpha in EAE (an animal model of multiple sclerosis) and after spinal cord injury, emphasizing the regulatory role of the BBB. As has been shown for CRH, saturable transport from brain to blood can exert effects in the periphery. Thus, the BBB plays a dynamic role in the communication of peptides between the periphery and the CNS.

A note on inconsistency of NPMLE of the distribution function from left truncated and case I interval censored data

We show that under reasonable conditions the nonparametric maximum likelihood estimate (NPMLE) of the distribution function from left-truncated and case 1 interval-censored data is inconsistent, in contrast to the consistency properties of the NPMLE from only left-truncated data or only interval-censored data. However, the conditional NPMLE is shown to be consistent. Numerical examples are provided to illustrate their finite sample properties.

Upregulation of tumor necrosis factor alpha transport across the blood-brain barrier after acute compressive spinal cord injury

Tumor necrosis factor alpha (TNF) is a cytokine that is involved in the inflammatory process after CNS injury and is implicated in neuroregeneration. A saturable transport system for TNF located at the blood-brain barrier (BBB) is responsible for the limited entry of TNF from blood to the CNS in normal mice. After partial disruption of the BBB by compression of the lumbar spinal cord, permeability to TNF was increased not only in the lumbar spinal cord but also in brain and distal spinal cord segments, where the BBB remained intact. The increase in the entry of TNF to the CNS followed a biphasic temporal pattern, with a first peak immediately after injury and a second peak starting on day 3; these changes lasted longer than the mere disruption of the BBB. The increased entry of TNF was abolished by addition of excess unlabeled TNF, showing that the transport system for TNF remained saturable after spinal cord injury (SCI) and providing evidence that the enhanced entry of TNF could not be explained by diffusion or leakage. This study adds strong support for our concept that the saturable transport system for TNF across the BBB can be upregulated in the diseased state, and it suggests that the BBB is actively involved in the modulation of the processes of degeneration and regeneration after SCI.

[Nucleotide sequence analysis for high variance region of hepatitis C virus in patients with hemodialysis]

OBJECTIVE

To investigate the route of transmission for hepatitis C virus (HCV) infection in patients on hemodialysis (HD).

METHODS

Nucleotide sequence of high variance region (HVR) of HCV was analyzed with nucleotide sequencing technique in 15 HD patients, and its homology was compared.

RESULTS

Comparison of the nucleotide sequences of HCV HVR in 15 HD patients showed that their homologies in isolates 8, 12 and 14 reached 97.50%, 95.00% in isolates 4, 5 and 10, and more than 80.00% in isolates 1, 3 - 8 and 10 - 14. Clinical data showed that all these patients were hemodialyzed in the same room with the same HD machine. Isolates 2 and all others were 61.25% - 66.25% in homology, and 57.50% - 67.50% in isolates 9 and all others, and both the patients had histories of large-quantity blood transfusion.

CONCLUSION

Blood transfusion was the primary risk factor for HCV infection in HD patients, but iatrogenic transmission of HCV through HD environment could also exist.

Analysis of site-specific phosphorylation of the retinoblastoma protein during cell cycle progression

Differential phosphorylation of the retinoblastoma protein plays a pivotal role in cell cycle regulation. The retinoblastoma protein is specifically phosphorylated during the cell cycle by cyclin-dependent kinase complexes which intersect with many cellular signaling networks. Since the loss of the retinoblastoma signaling pathways occurs in a wide variety of human tumors, understanding the significance of site-specific phosphorylation can clarify the role of selected cyclin-dependent kinase complexes during cell cycle progression. Here we describe the phosphospecificity and cellular characterization of a panel of polyclonal antibodies that recognize unique phosphorylation sites within the retinoblastoma protein. These reagents were used to validate authentic cellular retinoblastoma phosphorylation sites at amino acids 780, 795, and 807/811 correlating with the G1-S transition.

Penetration of neurotrophins and cytokines across the blood-brain/blood-spinal cord barrier

Now that peptides are no longer considered too large to cross the blood-brain barrier, attention has turned to the possibility that larger substances like polypeptides might also enter the central nervous system (CNS). This review summarizes evidence showing that many cytokines and neurotrophins not only enter the brain but also enter the spinal cord, sometimes faster than into the brain.

Growth inhibition by a triple ribozyme targeted to repetitive B2 transcripts

The B2 family represents a group of short repetitive sequences that are found throughout the rodent genome and are analogous to the human Alu sequences. Certain B2 subfamilies are transcribed by RNA polymerase III (pol III), and this transcription is in part controlled by the retinoblastoma protein. In addition to their putative role in retrotranspositional events, these actively transcribed B2 RNAs show a predicted highly stable secondary structure. Although B2 transcripts are normally confined to the nucleus, they demonstrate altered compartmentation after carcinogen treatment, in cancers, and in immortalized and/or transformed cell lines, the significance of which is unclear. Because modulation of B2 transcripts did not seem feasible with an antisense approach, we designed a triple ribozyme (TRz) construct to down-regulate B2 transcripts. The B2-targeted TRz undergoes efficient self-cleavage, resulting in liberation of the internal hammerhead Rz, which we targeted to a single-stranded region of the consensus B2 sequence. The liberated internal targeted Rz was 20 times more active than the corresponding double-G mutant construct that could not undergo self-cleavage, and 5 times more active than the same Rz flanked by nonspecific vector sequences. The B2-targeted TRz was used to develop stable transfectant clones from an SV40-immortalized hepatocyte cell line. These transfectant clones all showed variably reduced growth rates, accompanied by significant reductions in both cytoplasmic and nuclear B2 RNA levels: linear regression analyses showed that their growth rates were directly related to residual cytoplasmic B2 levels. Reverse-transcription polymerase chain reaction (RT-PCR) analyses documented efficient self-liberation of the internal targeted Rz in vivo, and showed that the relative cytoplasmic expression levels generally paralleled the magnitude of the decrease in B2 transcripts. The RT-PCR analyses further demonstrated that up to 20% of the Rz was located in the nucleus, which presumably reflects competition between autocatalytic processing and nucleocytoplasmic transport of the initial TRz transcript.

Tunable output power varying with the splitting ratio of a coupler from a fiber ring semiconductor laser

The tunable output power characteristics of a fiber ring semiconductor laser have been studied by use of rate equations. An expression for the tunable output power is deduced. The analysis confirms that there is an optimum coupler splitting ratio to trade off between wavelength tuning width and output power.

Saturable entry of ciliary neurotrophic factor into brain

Ciliary neurotrophic factor (CNTF), like tumor necrosis factor-alpha (TNF) and granulocyte-macrophage colony-stimulating factor (GM-CSF), is a cytokine with neurotrophic properties. Since all three cytokines are found in the periphery as well as brain, and since TNF and GM-CSF cross the blood-brain barrier (BBB) by a saturable mechanism, we investigated whether CNTF also saturably enters the brain from the blood. We found that CNTF crosses the BBB rapidly, with a rate of entry (Ki) of 4.60 (+/-0.78) x 10(-4) ml/g min, considerably faster than that of the 99mTc-albumin control. The Ki was reduced more than 3-fold by addition of excess unlabeled CNTF. The results indicate that CNTF is saturably transported across the BBB from blood to brain.

[Study of autoimmunity in progeny of pregnant women with systemic lupus erythematosus]

OBJECTIVE

To study the effect of systemic lupus erythematosus (SLE) on physical, mental development and plasma antibody level of SLE in their progenies.

METHODS

Routine physical examinations of 49 children from 48 SLE mothers were conducted. Compared immuno-fluorescence anti-nuclear antibody (IFANA), anticardiolipin (ACL), extractable nuclear antigen (ENA) and anti-ds-DNA plasma levels of SLE mothers and their progenies with that levels during pregnancy and in umbilical blood.

RESULTS

The physical development (height and weight) in 47 out of 49 children were within normal range while the remaining 2 were in the lower limit. The autoimmune antibodies were all negative in the umbilical blood with autoimmune negative mothers, while the anti-ribonucleoprotein (anti-RNP), anti-Smith surface antigen (anti-SSA), anti-specific soluble ribonucleic acid (anti-SSB) and ACL could be transferred to fetus through placenta. During follow up study, compared the autoimmune positive rates in progenies with that of mothers, the positive rates of IFANA and anti-ds-DNA decreased significantly (P < 0.01), while no changes in ACL. Compared the autoimmune positive rates in progenies with that of their own umbilical levels, the positive rates of IFANA, anti-RNP, anti-SSA decreased significantly (P < 0.01), while no difference existed in ACL. Boys showed faster disappearance of autoimmune positive rates than that of girls.

CONCLUSIONS

SLE did not show significant effects on the physical development of their progenies. Most autoimmune antibodies existed in umbilical blood were transferred through placenta during pregnancy and would disappear within 9 years after birth. Autoimmune antibodies decreased quicker in boys, and it indicated that girls should be follow-up more carefully. Autoimmune antibodies in the umbilical blood is an easy method for the screening of SLE progeny.

[Comparison of hypervariable region gene of hepatitis C virus between an individual infected persistently and an individual recovered from infection]

OBJECTIVE

To explore the law of recovery from HCV infection.

METHODS

Amplifiying, sequenceing and analyzing the gene of hypervariable region from an individual infected persistently and an individual recovered from infection.

RESULTS

One base insertion (two clones, 10%) in the person recovered other than that infected persistently, but there is no difference between them in percentage of mumation, diversity of quasispecies and genetic distance among clones.

CONCLUSION

The range of genetic variation at one time point did not seem to have obvious relationship with the HCV persistent infection or recovery from HCV infection. This study gave basic information for searching the relation of genetic variation and clinical status, and designing HCV vaccine.

Vaccine candidate MSP-1 from Plasmodium falciparum: a redesigned 4917 bp polynucleotide enables synthesis and isolation of full-length protein from Escherichia coli and mammalian cells

The Plasmodium falciparum malaria parasite is the causative agent of malaria tropica. Merozoites, one of the extracellular developmental stages of this parasite, expose at their surface the merozoite surface protein-1 complex (MSP-1), which results from the proteolytic processing of a 190-200 kDa precursor. MSP-1 is highly immunogenic in humans and numerous studies suggest that this protein is an effective target for a protective immune response. Although its function is unknown, there are indications that it may play a role during invasion of erythrocytes by merozoites. The parasite-derived msp-1 gene, which is approximately 5000 bp long, contains 74% AT. This high AT content has prevented stable cloning of the full-size gene in Escherichia coli and consequently its expression in heterologous systems. Here, we describe the synthesis of a 4917 bp gene encoding MSP-1 from the FCB-1 strain of P. falciparum adjusted for human codon preferences. The synthetic msp-1 gene (55% AT) was cloned, maintained and expressed in its entirety in E.coli as well as in CHO and HeLa cells. The purified protein is soluble and appears to possess native conformation because it reacts with a panel of mAbs specific for conformational epitopes. The strategy we used for synthesizing the full-length msp-1 gene was toassemble it from DNA fragments encoding all of the major proteolytic fragments normally generated at the parasite's surface. Thus, after subcloning we also obtained each of these MSP-1 processing products as hexahistidine fusion proteins in E.coli and isolated them by affinity chromatography on Ni2+agarose. The availability of defined preparations of MSP-1 and its major processing products open up new possibilities for in-depth studies at the structural and functional level of this important protein, including the exploration of MSP-1-based experimental vaccines.

Defining the substrate specificity of cdk4 kinase-cyclin D1 complex

cdk4 kinase-cyclin D1 complex (cdk4/D1) does not phosphorylate all of the sites within retinoblastoma protein (Rb) equally. Comparison of five phosphorylation sites within the 15 kDa C domain of Rb indicates that Ser795 is the preferred site of phosphorylation by cdk4/D1. A series of experiments has been performed to determine the properties of this site that direct preferential phosphorylation. For cdk4/D1, the preferred amino acid at the third position C-terminal to the phosphorylated serine/threonine is arginine. Substitution of other amino acids, including a conservative change to lysine, has dramatic effects on the rates of phosphorylation. This information has been used to mutate less favorable sites in Rb, converting them to sites that are now preferentially phosphorylated by cdk4/D1. A conserved site at Ser842 in the related pocket protein p107 is also preferentially phosphorylated by cdk4/D1. Although Rb and p107 differ significantly in sequence, the Rb Ser795 site can replace the p107 Ser842 site without affecting the rate of phosphorylation. These results suggest that although a determinant of specificity resides in the sequences surrounding the phosphorylated site, the structural context of the site is also a critical parameter of specificity.

Effects of peptides: a cross-listing of peptides and their central actions published in the journal Peptides from 1994 through 1998

Effects of peptides on the central nervous system are presented in two ways so as to provide a cross-listing. In the first table, the peptides are listed alphabetically. In the second table, the central nervous system effects are arranged alphabetically. No longer can there be any doubt that peptides affect the central nervous system, sometimes in several ways.

Uptake and degradation of blood-borne insulin by the olfactory bulb

Insulin found within the brain is derived from the blood and can affect various central nervous system (CNS) functions. The olfactory bulb contains one of the highest concentrations of insulin and insulin receptors within the CNS. To determine the mechanism underlying this high concentration of insulin, we used radioactively iodinated insulin to compare the blood to tissue transport rates and tissue degradation rates for the olfactory bulb, whole brain and spinal cord. We found that the olfactory bulb had both the highest transport rate across the blood-brain barrier (BBB) and the highest rate of degradation. Because a higher degradation rate would decrease, not increase, tissue concentrations of insulin, BBB transport may be the primary mechanism by which high concentrations of insulin are maintained within the olfactory bulb. This illustrates an adaptive aspect of the BBB in its regulation of the exchange of information molecules between the blood and the CNS.

Entry of EGF into brain is rapid and saturable

Epidermal growth factor (EGF) is a neurotrophic peptide produced both in the central nervous system and the periphery. Peripheral administration of EGF causes central nervous system-mediated changes. The central nervous system effects could be explained by the permeation of EGF across the blood-brain barrier (BBB). In this report, we show that 125I-EGF crosses the BBB rapidly, with an influx rate of about 2 microl/g x min, much faster than that for neurotrophins, cytokines, and most other bioactive peptides tested. The 125I-EGF was recovered intact in the brain 10 min after i.v. injection, and the majority of the peptide reaching the brain was present in the parenchyma. The fast rate of influx was significantly decreased by co-administration of nonradiolabeled EGF and transforming growth factor alpha, peptides that share the EGF receptor. By contrast, a monoclonal antibody against the EGF receptor failed to inhibit the entry of EGF. Furthermore, mice with a mutation in the EGF receptor had no significant decrease in the rapid rate of entry of 125I-EGF. By contrast to the fast rate of entry, 125I-EGF injected intracerebroventricularly (i.c.v.) only exited the brain with the bulk flow of cerebrospinal fluid. Thus, EGF has a saturable transport system at the BBB for rapid, unidirectional influx. The transport system does not require the entire EGF receptor and is susceptible to possible therapeutic manipulation.

Transport of brain-derived neurotrophic factor across the blood-brain barrier

Brain-derived neurotrophic factor (BDNF) is a potential therapeutic agent for degenerative disorders of the central nervous system. In this report, we investigated the ability of BDNF to cross the blood-brain barrier (BBB). BDNF was stable in blood up to 60 min after i.v. injection, with evidence for aggregation, and had an early, rapid influx into brain. By 10 min, most of the BDNF sequestered by the cerebral cortex was associated with the parenchyma rather than with the endothelial cells, demonstrating complete passage across the BBB. A small dose of unlabeled BDNF enhanced the entry of 125I-BDNF from blood to brain after an i.v. bolus injection, whereas larger doses had no effect. In contrast, a large dose of unlabeled BDNF inhibited the influx of 125I-BDNF during in situ brain perfusion. After intracerebroventricular injection, the efflux of BDNF from brain to blood occurred at a rate similar to that for reabsorption of cerebrospinal fluid, and no evidence for self-inhibition was found. Therefore, we conclude that intact BDNF in the peripheral circulation crosses the BBB by a high-capacity, saturable transport system.

Estimating survival curves with left-truncated and interval-censored data under monotone hazards

We show that the nonparametric maximum likelihood estimator (NPMLE) of a survival function may severely underestimate the survival probabilities at very early times for left-truncated and interval-censored data. As an alternative, we propose to compute the (nonparametric) MLE under a nondecreasing hazard assumption, the monotone MLE, by a gradient projection algorithm when the assumption holds. The projection step is accomplished via an isotonic regression algorithm, the pool-adjacent-violators algorithm. This gradient projection algorithm is computationally efficient and converges globally. Monte Carlo simulations show superior performance of the monotone MLE over that of the NPMLE in terms of either bias or variance, even for large samples. The methodology is illustrated with the application to the Wisconsin Epidemiological Study of Diabetic Retinopathy data to estimate the probability of incidence of retinopathy.

Usefulness of MRI in isolated upper cervical spine fractures in adults

A prospective analysis of patients admitted with isolated upper cervical spine fractures and who had magnetic resonance (MR) imaging performed within 48 h of the inciting traumatic event was completed to determine the clinical usefulness and cost effectiveness of routine MR screening. In patients with an identified neurologic deficit, MR findings changed the treatment of 25% (one of four) of the patients, whereas MR findings did not change the treatment of any patient identified without a neurologic deficit. We recommend that in adult patients with an isolated upper cervical spine fracture, MR should not be routinely ordered in patients without a neurologic deficit. This advanced imaging modality is not a useful or cost-effective screening device for patients presenting with a fracture of the upper cervical spine without neurologic deficit.

Triple ribozyme-mediated down-regulation of the retinoblastoma gene

We have been developing triple ribozyme (TRz) constructs which consist of two cis-acting ribozymes flanking an internal trans-acting ribozyme, which is targeted to a cellular RNA. Actions of the two cis-acting ribozymes efficiently liberate the internal ribozyme with minimal non-specific flanking sequences. The liberated internal targeted ribozyme shows substantially greater catalytic activity than TRz preparations, constructs which cannot undergo self-liberation or than single ribozymes with flanking vector sequences. Here we construct a TRz which was targeted to retinoblastoma gene (Rb) mRNA, which cleaved Rb target RNA in vitro as expected. A number of tetracycline-regulatable clones stably transfected with the Rb-targeted TRz were developed and analyzed. The internal targeted ribozymes were efficiently liberated in vivo and the stably transfected clones showed varied reductions in Rb mRNA, which were contingent upon ribozyme expression and catalytic activity. The two clones showing major reductions in Rb mRNA (and pRb) levels (>70% reduction) showed abnormal morphology, loss of contact inhibition and the ability to grow in soft agar, as well as altered compartmentation of repetitive B2 transcripts, a phenomenon previously associated with immortalization and/or transformation. TRz constructs coupled with tissue-specific promoters should allow development of in vivo models in which Rb function is markedly reduced in a tissue-specific manner.

[A case-control study on risk factors of periodontitis]

A 1:1 matched case-control study was conducted to determine the risk factors of periodontitis for the government cadres in Wuhan city. Results showed that dental calculus, cigarette smoking and alcohol consumption were identified as risk factors of periodontitis with odds ratios 5.31, 2.13 and 1.86 respectively. Drinking tea was not found related with periodontitis. In the process of periodontitis, synergetic effect between cigarette smoking, alcohol drinking and dental calculus were noticed. In the general population, the proportions of periodontitis cases attributed to alcohol drinking, cigarette smoking and dental calculus were 25.12%, 24.04% and 46.29% respectively.

Delta subunit of rat liver mitochondrial ATP synthase: molecular description and novel insights into the nature of its association with the F1-moiety

The F1 moiety of ATP synthase complexes consists of five subunit types in the stoichiometric ratio alpha 3, beta 3, gamma, delta epsilon. Of these, the delta subunit has received very little attention in the study of F1 preparations from eukaryotic cells. Although recently shown to associate tightly with the beta subunit [Pedersen, P. L., Hullihen, J., Bianchet, M., Amzel, L. M., and Lebowitz, M. S. (1995) J. Biol. Chem. 270, 1775-1784], the delta subunit is not resolved in the X-ray structure of either the rat liver or bovine heart enzyme. For these reasons, the novel studies reported here were designed both to provide a molecular description of the rat liver delta subunit and to gain insight into the nature of its interaction with F1. The rat liver delta subunit was cloned from a lambda gt11 library, sequenced, overexpressed in Escherichia coli (E. coli) in fusion with the maltose binding protein, and, after cleavage of the latter protein, purified to homogeneity. The purified delta subunit (MW = 14.7 kDa) was shown by circular dichroism spectroscopy to be highly structured and to exhibit about 25% sequence identity to the chloroplast and E. coli epsilon subunits, frequently regarded as homologues of higher eukaryotic delta subunits. Significantly, and in contrast to the chloroplast and E. coli epsilon subunits, which are readily removed from their parent F1 moieties after treatment respectively with ethanol and lauryldimethylamine oxide, the rat liver delta subunit remained tightly bound to F1 under these relatively mild conditions. For the above reasons, four types of experiments were carried out on rat liver F1 in order to (1) determine the accessibility of the delta subunit to both specific antibodies and to proteases, (2) establish the effect of nucleotides on this subunit's accessibility, (3) identify in cross-linking studies with disuccinimidyl glutarate this subunit's most reactive neighbor, and (4) determine whether this subunit can be dissociated from F1 by using ionic detergents while leaving the remaining complex intact. The data derived from this detailed set of studies (a) supports the view that the rat liver F1-delta subunit is in very close proximity to the gamma subunit near the bottom of the F1 molecule but does not penetrate deeply into the central core, (b) shows that within F1 the delta subunit's N-terminus is exposed while its C-terminus is masked, (c) indicates that access to the delta subunit is shielded in part by the alpha, beta, and gamma subunits and changes during the catalytic cycle of F1, and (d) implicates the delta subunit as important for the structural stability of the F1 unit. These novel findings on a higher eukaryotic F1-delta subunit are discussed in relationship to earlier studies on the related epsilon subunits from both chloroplasts and E. coli.