Mortality Burden Due to Short-term Exposure to Fine Particulate Matter in Korea

OBJECTIVES

Excess mortality associated with long-term exposure to fine particulate matter (PM2.5) has been documented. However, research on the disease burden following short-term exposure is scarce. We investigated the cause-specific mortality burden of short-term exposure to PM2.5 by considering the potential non-linear concentration-response relationship in Korea.

METHODS

Daily cause-specific mortality rates and PM2.5 exposure levels from 2010 to 2019 were collected for 8 Korean cities and 9 provinces. A generalized additive mixed model was employed to estimate the non-linear relationship between PM2.5 exposure and cause-specific mortality levels. We assumed no detrimental health effects of PM2.5 concentrations below 15 μg/m3. Overall deaths attributable to short-term PM2.5 exposure were estimated by summing the daily numbers of excess deaths associated with ambient PM2.5 exposure.

RESULTS

Of the 2 749 704 recorded deaths, 2 453 686 (89.2%) were non-accidental, 591 267 (21.5%) were cardiovascular, and 141 066 (5.1%) were respiratory in nature. A non-linear relationship was observed between all-cause mortality and exposure to PM2.5 at lag0, whereas linear associations were evident for cause-specific mortalities. Overall, 10 814 all-cause, 7855 non-accidental, 1642 cardiovascular, and 708 respiratory deaths were attributed to short-term exposure to PM2.5. The estimated number of all-cause excess deaths due to short-term PM2.5 exposure in 2019 was 1039 (95% confidence interval, 604 to 1472).

CONCLUSIONS

Our findings indicate an association between short-term PM2.5 exposure and various mortality rates (all-cause, non-accidental, cardiovascular, and respiratory) in Korea over the period from 2010 to 2019. Consequently, action plans should be developed to reduce deaths attributable to short-term exposure to PM2.5.

Risk of Cancer in Middle-aged Patients With Gout: A Nationwide Population-based Study in Korea

OBJECTIVE

Gout is reportedly associated with a higher incidence of cancer. However, patients with gout tend to have several cancer-related factors including obesity, smoking, and alcohol consumption; thus, the precise association between gout and cancer risk remains unclear. We aimed to investigate the risk of cancer in Korean patients with gout.

METHODS

Based on the Korea Health Insurance Service database, the subjects comprised patients aged 41-55 years with gout newly diagnosed between 2003 and 2007. We used a multivariable-adjusted Cox proportional hazards model in gout patients and a 1:2 ratio for the matched controls by age, sex, and index year.

RESULTS

We compared 4176 patients with gout with 8352 controls. The mean age and follow-up duration were 48.8 years and 10.1 years in both groups. Overall cancer risk was significantly different between gout patients and controls (HR 1.224, 95% CI 1.073-1.398). The all-cause mortality (HR 1.457, 95% CI 1.149-1.847) and cancer mortality (HR 1.470, 95% CI 1.020-2.136) were higher in patients with gout. In the subgroup analysis, the cancer risks of the stomach (HR 1.710, 95% CI 1.221-2.395), head and neck (HR 1.850, 95% CI 1.071-3.196), and hematologic or lymphoid organ (HR 2.849, 95% CI 1.035-7.844) were higher in patients with gout.

CONCLUSION

Patients aged 41-55 years with gout have a higher risk of cancer and all-cause and cancer mortality compared with the general population. Therefore, special attention should be paid to higher cancer risk and mortality in these patients who are diagnosed in middle age.

Cancer incidence according to the National Health Information Database in Korean patients with end-stage renal disease receiving hemodialysis

BACKGROUND/AIMS

The aim of this study was to investigate incidence, survival, and risk factors of cancer in end-stage renal disease (ESRD) patients with hemodialysis using information from the National Health Information Database (NHID).

METHODS

Using the NHID, we identified ESRD patients who started maintenance hemodialysis between 2003 and 2005 in Korea. Patients were followed from initiation of hemodialysis to renal transplantation, death, or December 31, 2016, whichever came first. We calculated the incidence, survival, and risk factors of cancer.

RESULTS

Of the total 14,382 ESRD patients, 1,124 (7.82%; men:women, 728:396) were diagnosed with cancer during follow-up. The mean duration from the start of hemodialysis to new cancer identification was 64.40 ± 41.81 months. Significant risk factors for the development of new cancer were old age, male sex, and liver disease. Conversely, patients with diabetes showed low risk for new cancer. The colorectum (17.31%) was the most common primary site of cancer in men, followed by the liver (15.8%), stomach (14.29%), lung (13.6%), and kidney (10.3%). In women, the colorectum (14.65%) was also the most common primary site of cancer, followed by the breast (12.88%), thyroid (12.63%), stomach (10.86%), and lung (8.08%). According to the primary site of cancer, breast cancer showed the longest median survival duration (130.93 months), followed by thyroid, kidney, colorectum, bladder, stomach, liver, and lung cancer. On multivariate analyses, overall survival was affected by age and diabetes.

CONCLUSION

The cancer incidence of chronic hemodialysis patients was relatively high. Thus, careful monitoring and a specific cancer screening program are needed for chronic hemodialysis patients.

Reduced liver cancer mortality with regular clinic follow-up among patients with chronic hepatitis B: A nationwide cohort study

Background

Regular clinic follow‐up is a prerequisite for optimal antiviral therapy and surveillance of hepatocellular carcinoma in patients with chronic hepatitis B (CHB). However, adherence to regular follow‐up stays low in practice. This study investigated whether regular follow‐up is associated with decreased liver cancer mortality in CHB patients.

Methods

A nationwide population‐based historical cohort study was conducted using customized data from the National Health Insurance Service of Korea. The number of hospital visits every 3‐month interval was counted for 2 years from the date of CHB diagnosis. Patients were classified into three follow‐up groups: regular (four to eight visits), irregular (one to three visits), and no follow‐up. The risk of liver cancer mortality was compared among the groups using Cox proportional hazard regression analysis.

Results

Of the 414 074 CHB patients, 22.9% had regular follow‐up. In multivariable analysis, regular follow‐up was independently associated with decreased risk of liver cancer mortality compared to no follow‐up (hazard ratio [HR], 0.56; 95% confidence interval [CI], 0.50‐0.63, P < .001). Regular follow‐up was also associated with the lowest risk of all‐cause mortality (HR, 0.60; 95% CI, 0.57‐0.63, P < .001). Patients with regular follow‐up received more curative treatment (23.1% vs 15.1%, P < .001). Patients were less motivated when they were female, >60 years, of low socioeconomic status, disabled, lived in a rural area, had a higher comorbidity rate, or did not have cirrhosis.

Conclusions

Regular follow‐up at least every 3‐6 months is significantly associated with reduced liver cancer mortality in patients with CHB.

Independent effect of physical activity and resting heart rate on the incidence of atrial fibrillation in the general population

While physical activity (PA) may influence resting heart rate (RHR), and a low RHR may be a risk factor for atrial fibrillation (AF), controversy exists regarding the association between PA and development of AF. Using data from a Korean, prospective population cohort, we investigated the independent effect of PA and RHR on the incidence of AF in the general population. A total of 8,811 participants aged 40-69 years were analyzed. Total PA assessed based on questionnaires was divided into quartiles, with the lowest to the highest being Q1, Q2, Q3, and Q4. During a median follow-up of 139 months, AF developed in 167 participants (1.9%). Q3 of total PA was associated with a significantly lower risk of AF than Q1 even after adjusting for RHR as a covariate, but Q4 was not. The risk of AF was higher in participants with RHR < 60 bpm than in those with RHR 70-85 bpm, and the significance persisted after adjusting for PA as a covariate. This study showed that a moderate amount of total PA was associated with a lower risk of incident AF independent of RHR and that low RHR was an independent risk factor for AF in the general Korean population.

Incidence and Prevalence of Idiopathic Inflammatory Myopathies in Korea: a Nationwide Population-based Study

BACKGROUND

This study aimed to estimate the incidence and prevalence of idiopathic inflammatory myopathies (IIM) and associated comorbidities in Korea from 2006 to 2015.

METHODS

IIM between 2004 to 2015 were identified using the Korean National Health Insurance Service medical claim database. The case definition required more than one visit based on diagnostic codes including juvenile dermatomyositis (JDM), dermatomyositis (DM), or polymyositis (PM) and registration in the Individual Copayment Beneficiaries Program (ICBP) for rare and intractable diseases. IIM patients with a disease-free period of 24 months before the index date were defined as incident cases. The Elixhauser comorbidity score was calculated.

RESULTS

Using the base case definition, 1,150 prevalent patients with IIM (117 JDM, 521 DM, 512 PM) were recorded in 2006 and 2,210 (130 JDM, 1,101 DM, 869 PM) in 2015. The prevalence was estimated at 2.3-4.0 (0.9-1.2 for JDM, 1.2-2.7 for DM, 1.4-2.1 for PM)/100,000 person-year (PY). We identified 218 incident cases of IIM in 2006 (18 JDM, 98 DM, 102 PM) and 191 cases (7 JDM, 83 DM, 101 PM) in 2015. The incidence was estimated at 2.9-5.2 (0.7-1.9 for JDM, 1.8-4.0 for DM, 1.6-3.0 for PM)/1,000,000 PY. The mean age (± standard deviation) of prevalent patients with IIM was 51.2 (± 16.9) years, and the percentage of women was 72.1%. More than two-thirds of patients (70.7%) had more than two comorbidities. Twenty percent of patients had interstitial lung diseases.

CONCLUSION

In Korea, the incidence and prevalence of IIM were 2.9-5.2/1,000,000 PY and 2.3-4.0/100,000 PY, respectively.

Antihistamine use and the risk of injurious falls or fracture in elderly patients: a systematic review and meta-analysis

Despite their anticholinergic side effects, first-generation antihistamines are widely prescribed to elderly patients. A systematic review was conducted to synthesize real-world evidence. First-generation antihistamine use is considerably associated with an increased risk of injurious falls or fracture among the elderly.

INTRODUCTION

First-generation antihistamines are considered potentially inappropriate for elderly patients owing to anticholinergic side effects. We aimed to determine whether elderly patients taking antihistamines are at increased risk of injurious falls or fracture.

METHODS

We identified studies in MEDLINE, EMBASE, and several local databases through November 2016. Observational studies on the association between antihistamine use and the risk of injurious falls or fracture were selected. Quality of the studies and the level of evidence were assessed. The random-effects model was employed for meta-analysis, and heterogeneity was examined based on I-square and Cochrane's Q test. Subgroup analyses were performed when the heterogeneity among studies could not be explained.

RESULTS

From 473 identified studies, five (three case-control studies, one cohort study, and one case-crossover study) were included in our analysis based on eligibility criteria. First-generation antihistamine use showed significantly increased risk of injurious falls or fracture (odds ratio [OR] 2.03, 95% confidence interval [CI] 1.49-2.76, heterogeneity: p = 0.41, I² = 0%). Studies including antihistamines of all generations or containing no generation information were dealing with falls during hospitalization. Among these studies, the association was statistically significant without heterogeneity (OR 2.89, 95% CI 1.71-4.89, heterogeneity: p = 0.42, I² = 0%). Due to the small number of studies included and unadjusted results, meaningful interpretation based on subgroup analysis was limited.

CONCLUSIONS

First-generation antihistamine use is considerably associated with increased risk of injurious falls or fracture among the elderly. Clinicians need to exercise caution when prescribing first-generation antihistamines to elderly patients.

The ubiquitin-proteasome pathway and proteasome inhibitors

The ubiquitin-proteasome pathway has emerged as a central player in the regulation of several diverse cellular processes. Here, we describe the important components of this complex biochemical machinery as well as several important cellular substrates targeted by this pathway and examples of human diseases resulting from defects in various components of the ubiquitin-proteasome pathway. In addition, this review covers the chemistry of synthetic and natural proteasome inhibitors, emphasizing their mode of actions toward the 20S proteasome. Given the importance of proteasome-mediated protein degradation in various intracellular processes, inhibitors of this pathway will continue to serve as both molecular probes of major cellular networks as well as potential therapeutic agents for various human diseases.

Inducible model to study negative strand RNA synthesis and assembly of hepatitis C virus from a full-length cDNA clone

An inducible in vitro cell culture system was developed to assay HCV replication by direct biochemical means. A transcription plasmid containing a T7 promoter at the 5' end, full-length cDNA of the HCV genome, a ribozyme sequence from the antigenomic strand of hepatitis delta virus and a T7 terminator was prepared. To facilitate high-level transcription of HCV RNA, HepG2 cells were infected with replication deficient adenovirus containing the T7 RNA polymerase gene and later transfected with the transcription plasmid containing the full-length HCV genome. This transfection-based cell culture system expressed high levels of HCV structural (core, El and E2) and non-structural proteins (NS3 and NS5B) detectable by Western blot and immunofluorescence assays. Production of HCV RNA transcripts and presence of replicative negative strand of HCV was confirmed by ribonuclease protection assay indicating replication of HCV in the transfected HepG2 cell. The transfected HepG2 cells assembled 50-60 nm virus-like particles, which could be aggregated by anti-E2 antibodies. This model can be utilized for studying mechanisms of HCV replication, assembly of HCV particles and to test potential anti-HCV compounds.

Lack of proteasome active site allostery as revealed by subunit-specific inhibitors

The chymotrypsin-like (CT-L) activity of the proteasome is downregulated by substrates of the peptidyl-glutamyl peptide hydrolyzing (PGPH) activity. To investigate the nature of such interactions, we synthesized selective alpha',beta'-epoxyketone inhibitors of the PGPH activity. In cellular proliferation and protein degradation assays, these inhibitors revealed that selective PGPH inhibition was insufficient to inhibit protein degradation, indicating that the CT-L and PGPH sites function independently. We also demonstrated that CT-L inhibition by a PGPH substrate does not require the occupancy of the PGPH site or hydrolysis of the PGPH substrate. Thus, these results support a model in which a substrate of one subunit regulates the activity of another via binding to a noncatalytic site(s) rather than through binding to an active site.

Cells adapted to the proteasome inhibitor 4-hydroxy- 5-iodo-3-nitrophenylacetyl-Leu-Leu-leucinal-vinyl sulfone require enzymatically active proteasomes for continued survival

The proteasome is the primary protease used by cells for degrading proteins and generating peptide ligands for class I molecules of the major histocompatibility complex. Based on the properties of cells adapted to grow in the presence of the proteasome inhibitor 4-hydroxy-5-iodo-3-nitrophenylacetyl-Leu-Leu-leucinal-vinyl sulfone (NLVS), it was proposed that proteasomes can be replaced by alternative proteolytic systems, particularly a large proteolytic complex with a tripeptidyl peptidase II activity. Here we show that NLVS-adapted cells retain sensitivity to a number of highly specific proteasome inhibitors with regard to antigenic peptide generation, accumulation of polyubiquitinated proteins, degradation of p53, and cell viability. In addition, we show that in the same assays (with a single minor exception), NLVS-adapted cells are about as sensitive as nonselected cells to Ala-Ala-Phe-chloromethylketone, a specific inhibitor of tripeptidyl peptidase II activity. Based on these findings, we conclude that proteasomes still have essential proteolytic functions in adapted cells that are not replaced by Ala-Ala-Phe-chloromethylketone-sensitive proteases.

HCV RNA levels in hepatocellular carcinomas and adjacent non-tumorous livers

To determine the antiviral effects of drugs targeted to hepatitis C virus (HCV) in chronic hepatitis patients, an accurate quantitative method with high sensitivity is needed. Reverse transcription nested polymerase chain reaction (RT-PCR) is the most sensitive method for the detection of HCV sequences in clinical specimens. However, this method is not quantitative. For this purpose, a quantitative competitive assay was developed that combines RT and PCR followed by image analysis to quantify HCV RNA. This assay targets the highly conserved 5' non-coding region of HCV and is based on the co-amplification of wild type HCV RNA with known amounts of mutant synthetic RNA. The mutant internal control used in these experiments differs from the wild type RNA by two nucleotide substitutions, which introduces an internal restriction enzyme site. In this report, this method was used to determine the levels of positive strand RNA in 11 HCV positive hepatocellular carcinomas (HCC) and compared these with adjacent non-tumorous liver tissue. To confirm that the difference in viral titers is not related to variations in the amount of RNA used in the assay, glyceraldehyde 3-phosphate dehydrogenase (GAPDH) mRNA was also assessed by competitive RT-PCR in all tissue extracts. Using this competitive assay it was determined that HCV RNA levels in the liver and tumor samples ranged from 10(3) to 10(6) molecules per microg of total RNA which is similar to previous reports. Interestingly, the amount of HCV in all the non-tumorous liver specimens were found to be significantly higher (P<0.05) than the surrounding tumors, while the GAPDH mRNA levels were found to be similar in both liver and tumor. Competitive RT-PCR is a sensitive, accurate and reliable method to determine HCV titers in clinical specimens. Using this method it was determined that malignant tumor cells harbor less HCV as compared with the surrounding non-tumorous liver cells.

Proteasome inhibition by the natural products epoxomicin and dihydroeponemycin: insights into specificity and potency

While two structurally related epoxyketone-containing antitumor natural products, epoxomicin and eponemycin, share the proteasome as a common intracellular target, they differ in their antiproliferative activity, proteasome subunit binding specificity, and rates of proteasome inhibition. As a first step towards understanding such differences and developing novel proteasome subunit-specific inhibitors, we report here the synthesis and characterization of epoxomicin/dihydroeponemycin chimerae.

Towards subunit-specific proteasome inhibitors: synthesis and evaluation of peptide alpha',beta'-epoxyketones

BACKGROUND

The proteasome is a large multicatalytic protease complex (700 kDa) involved in a number of highly regulated processes. It has three major catalytic activities: a chymotrypsin-like activity, a trypsin-like activity and a post-glutamyl peptide hydrolyzing (PGPH) activity. To be useful as molecular probes, which could help dissect the cellular functions of the proteasome, inhibitors should be specific for the proteasome, active in vivo and selectively block only one of the three catalytic activities. To date, few inhibitors fulfill these requirements so we set out to make novel proteasome inhibitors that incorporate these characteristics.

RESULTS

A panel of amino-terminally acetylated peptide alpha',beta'-epoxyketones with leucine in P1 and various aliphatic or aromatic amino acids in P2-P4 were prepared and evaluated. Most compounds selectively inhibited the chymotrypsin-like activity, while only weakly inhibiting the trypsin-like and PGPH activities. After optimization, one inhibitor, Ac-hFLFL-epoxide, was found to be more potent and selective for the inhibition of the chymotrypsin-like activity than several previously described inhibitors. This inhibitor also exhibited strong in vivo anti-inflammatory activity.

CONCLUSIONS

Optimization of amino-terminally acetylated peptide alpha',beta'-epoxyketones furnished a potent proteasome inhibitor, Ac-hFLFL-epoxide, that has an excellent selectivity for the chymotrypsin-like activity. The inhibitor also proved to be a potent antiproliferative and anti-inflammatory agent. The strong in vivo and in vitro activities suggest that this class of proteasome inhibitors could be both molecular probes and therapeutic agents.

There is only one phosphoenzyme intermediate with bound calcium on the reaction pathway of the sarcoplasmic reticulum calcium ATPase

Identical first-order rate constants for phosphorylation of the calcium ATPase of sarcoplasmic reticulum by bound inorganic phosphate (Pi) of 25 +/- 2 s-1 with empty vesicles and 25 +/- 1 s-1 with vesicles that were passively loaded with 40 mM Ca2+ were obtained by treating the reaction as an approach to equilibrium (4 mM [32P]Pi, 20 mM MgCl2, 10 mM EGTA, and 100 mM KCl at pH 7.0 and 25 degrees C). The formation of ADP-sensitive phosphoenzyme from Pi with Ca(2+)-loaded vesicles also proceeds with a first-order rate constant of 25 s-1 and no detectable induction period. These identical rate constants show that lumenal Ca2+ does not inhibit the rate of phosphorylation of the enzyme by bound Pi and that there is no significant kinetic barrier for the conformational change that converts an ADP-insensitive to an ADP-sensitive phosphoenzyme intermediate with bound Ca2+. We conclude that there is no evidence for the existence of two stable phosphoenzyme intermediates with bound Ca2+, such as E1 approximately P.Mg.Ca2 and Ca2.E2-P.Mg, that are included in the E1-E2 and related two-state models for calcium transport by this enzyme. In general, coupling of a physical reaction, such as muscle contraction or vectorial transport, to a chemical reaction, such as ATP hydrolysis, requires more than two states in the reaction cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate specificity for catalysis of phosphoryl transfer by the calcium ATPase of sarcoplasmic reticulum

When alpha,beta-methylene ADP (alpha,beta-CH2-ADP) is added to the phosphorylated calcium ATPase of sarcoplasmic reticulum with Ca(2+)-bound, Ca2.E approximately P.Mg, alpha,beta-methylene ATP is not synthesized (5 mM MgCl2, 100 mM KCl, pH 7.0, 25 degrees C). Similarly, adenosine 5'-O-(2-thiotriphosphate) is not synthesized from reaction of the phosphoenzyme with adenosine 5'-O-(2-thiodiphosphate), ADP beta S. In contrast, ATP is formed rapidly and reversibly from the reaction of the phosphoenzyme with ADP. Both ADP analogs are competitive inhibitors for the binding of ADP to the phosphoenzyme with KADPS = 0.45 mM: alpha,beta-CH2-ADP and ADP beta S bind to the phosphoenzyme with K alpha,beta-CH2-ADPS = 0.92 mM and KADP beta SS = 0.05 mM, respectively. We conclude that phosphoryl transfer from the phosphoenzyme to alpha,beta-CH2-ADP is kinetically blocked, although it is thermodynamically favorable. The rate acceleration of > 10(5) for phosphoryl transfer from Ca2.E approximately P.Mg to ADP compared to alpha,beta-CH2-ADP can be attributed to the differences in both the structure and the net charge of ADP compared with alpha,beta-CH2-ADP at pH 7.0. Phosphoryl transfer from the phosphoenzyme to ADP beta S is thermodynamically so unfavorable that we cannot determine whether the transition state is also unfavorable.

Lumenal and cytoplasmic binding sites for calcium on the calcium ATPase of sarcoplasmic reticulum are different and independent

The calcium ATPase of sarcoplasmic reticulum reacts with inorganic phosphate (Pi) to form phosphoenzyme that can bind two Ca2+ ions from the lumen of intact vesicles. Therefore, as the concentration of lumenal Ca2+ is increased, the concentration of phosphoenzyme at equilibrium increases; however, it levels off at lower maximal concentrations with decreasing concentrations of Pi. This requires that two Ca2+ ions can bind to lumenal binding sites of both the phosphoenzyme and the unphosphorylated enzyme. If lumenal Ca2+ could bind only to the phosphoenzyme, saturating concentrations of lumenal Ca2+ would drive phosphoenzyme formation to completion even at low concentrations of Pi. Phosphorylation is inhibited by cytoplasmic Ca2+ with K0.5 = 2.1 and 4 microM in the absence and in the presence of 40 mM lumenal Ca2+, respectively. K0.5 = 4 microM is much lower than K0.5 = 70 microM, which is expected if lumenal Ca2+ could bind only to the phosphoenzyme. Occupancy of the lumenal sites on the unphosphorylated enzyme by Ca2+ does not significantly change the rate constants of kp = 220 s-1 for phosphorylation by ATP, kCa = 90 s-1 for dissociation of Ca2+, and kMg = 50 s-1 for dissociation of Mg2+. We conclude that the calcium ATPase has two low-affinity lumenal Ca(2+)-binding sites that are independent of the high-affinity cytoplasmic Ca(2+)-binding sites. The results are consistent with a mechanism of Ca2+ transport in which phosphorylation of the enzyme by ATP drives the translocation of two Ca2+ ions from the high-affinity to the low-affinity sites.

Calcium ATPase of sarcoplasmic reticulum has four binding sites for calcium

The calcium-transporting ATPase of sarcoplasmic reticulum is known to bind two Ca2+ ions from the cytoplasm to the free enzyme and two Ca2+ ions from the lumen to the phosphoenzyme. The concentration of phosphoenzyme formed at equilibrium from Pi and Mg2+ increases with increasing concentration of calcium in the lumen, which binds to the phosphoenzyme to form Ca2.E approximately P.Mg. However, at subsaturating concentrations of Mg2+ increasing the concentration of lumenal Ca2+ does not drive phosphoenzyme formation to completion. The maximal levels of phosphoenzyme that are formed at saturating concentrations of lumenal Ca2+ increase with increasing concentrations of Mg2+. This result requires that Ca2+ can bind to low-affinity lumenal sites on both the free enzyme and the phosphoenzyme, as well as to the high-affinity cytoplasmic calcium-binding sites. If there were no lumenal binding sites for Ca2+ on the free enzyme, high concentrations of lumenal Ca2+ would convert all of the enzyme to the same maximal concentration of Ca2.E approximately P.Mg at subsaturating concentrations of Mg2+ and Pi. We conclude that there are two low-affinity lumenal sites as well as two high-affinity cytoplasmic sites for Ca2+ on the free enzyme. Phosphorylation by ATP results in translocation of Ca2+ from the high-affinity to the low-affinity sites.

The vectorial specificity for calcium binding to the CaATPase of sarcoplasmic reticulum is controlled by phosphorylation, not by an E-E* conformational change

The E-E* model for calcium pumping by the CaATPase of sarcoplasmic reticulum includes two distinct conformational states of the enzyme, E and E*. Exterior Ca2+ binds only to E and interior Ca2+ binds only to E*. Therefore, it is expected that there will be competition between the binding of calcium to the unphosphorylated enzyme from the two sides of the membrane. The equilibrium concentration of cECa2, the enzyme with Ca2+ bound at the exterior site, was measured at different Ca2+ concentrations with empty sarcoplasmic reticulum vesicles (SRV) and with SRV loaded with 40 mM Ca2+ by reaction with 0.5 mM [gamma-32P]ATP plus 20 mM EGTA for 13 ms (100 mM KCl, 5 mM MgSO4, 40 mM Mops/KOH, pH 7.0, 25 degrees C). The sigmoidal dependence on free exterior calcium concentration of the concentration of cECa2, measured as [32P]phosphoenzyme, is identical with empty and loaded SRV, within experimental error. The value of K0.5 is 2.8 microM, and the Hill coefficient is 2. This result shows that there is no competition between binding of Ca2+ to the outside and the inside of the membrane. This is consistent with a model in which the vectorial specificity for calcium binding is controlled by the chemical state of the enzyme, rather than a simple conformational change. It is concluded that there are not two interconverting forms of the free enzyme, E and E*, instead the vectorial specificity for binding and dissociation of Ca2+ is determined by the state of phosphorylation of the CaATPase.

The calcium ATPase of sarcoplasmic reticulum is inhibited by one Ca2+ ion

Inhibition by calcium of the steady-state turnover of the calcium ATPase from sarcoplasmic reticulum of rabbit muscle follows a Hill slope of 0.8 +/- 0.2 (pH 7.0, 0.1 M KCl, varying [Mg2+] and 2 microM A23187 ionophore). It is concluded that dissociation of the two Ca2+ ions from E-P.Ca2 is sequential and that the inhibition arises from the binding of one Ca2+ to A-P.Ca1.