Nm23/NDP kinases in hepatocellular carcinoma

Mechanisms of action of NME metastasis suppressors - a family affair

Metastatic progression is regulated by metastasis promoter and suppressor genes. NME1, the prototypic and first described metastasis suppressor gene, encodes a nucleoside diphosphate kinase (NDPK) involved in nucleotide metabolism; two related family members, NME2 and NME4, are also reported as metastasis suppressors. These proteins physically interact with members of the GTPase dynamin family, which have key functions in membrane fission and fusion reactions necessary for endocytosis and mitochondrial dynamics. Evidence supports a model in which NDPKs provide GTP to dynamins to maintain a high local GTP concentration for optimal dynamin function. NME1 and NME2 are cytosolic enzymes that provide GTP to dynamins at the plasma membrane, which drive endocytosis, suggesting that these NMEs are necessary to attenuate signaling by receptors on the cell surface. Disruption of NDPK activity in NME-deficient tumors may thus drive metastasis by prolonging signaling. NME4 is a mitochondrial enzyme that interacts with the dynamin OPA1 at the mitochondria inner membrane to drive inner membrane fusion and maintain a fused mitochondrial network. This function is consistent with the current view that mitochondrial fusion inhibits the metastatic potential of tumor cells whereas mitochondrial fission promotes metastasis progression. The roles of NME family members in dynamin-mediated endocytosis and mitochondrial dynamics and the intimate link between these processes and metastasis provide a new framework to understand the metastasis suppressor functions of NME proteins.

Extracellular Vesicle-Mediated Metastasis Suppressors NME1 and NME2 Modify Lipid Metabolism in Fibroblasts

Simple Summary

Communication between cancer and stromal cells involves paracrine signalling mediated by extracellular vesicles (EVs). EVs transmit essential factors among cells of the tumour microenvironment. EVs derived from both cancer and stromal cells have been implicated in tumour progression. In this study, we focused on the first identified metastasis suppressor NME1, and on its close homolog NME2, and investigated their function in EVs in the interplay between cancer and stromal cells.

Abstract

Nowadays, extracellular vesicles (EVs) raise a great interest as they are implicated in intercellular communication between cancer and stromal cells. Our aim was to understand how vesicular NME1 and NME2 released by breast cancer cells influence the tumour microenvironment. As a model, we used human invasive breast carcinoma cells overexpressing NME1 or NME2, and first analysed in detail the presence of both isoforms in EV subtypes by capillary Western immunoassay (WES) and immunoelectron microscopy. Data obtained by both methods showed that NME1 was present in medium-sized EVs or microvesicles, whereas NME2 was abundant in both microvesicles and small-sized EVs or exosomes. Next, human skin-derived fibroblasts were treated with NME1 or NME2 containing EVs, and subsequently mRNA expression changes in fibroblasts were examined. RNAseq results showed that the expression of fatty acid and cholesterol metabolism-related genes was decreased significantly in response to NME1 or NME2 containing EV treatment. We found that FASN (fatty acid synthase) and ACSS2 (acyl-coenzyme A synthetase short-chain family member 2), related to fatty acid synthesis and oxidation, were underexpressed in NME1/2-EV-treated fibroblasts. Our data show an emerging link between NME-containing EVs and regulation of tumour metabolism.

NME/NM23/NDPK and Histidine Phosphorylation

The NME (Non-metastatic) family members, also known as NDPKs (nucleoside diphosphate kinases), were originally identified and studied for their nucleoside diphosphate kinase activities. This family of kinases is extremely well conserved through evolution, being found in prokaryotes and eukaryotes, but also diverges enough to create a range of complexity, with homologous members having distinct functions in cells. In addition to nucleoside diphosphate kinase activity, some family members are reported to possess protein-histidine kinase activity, which, because of the lability of phosphohistidine, has been difficult to study due to the experimental challenges and lack of molecular tools. However, over the past few years, new methods to investigate this unstable modification and histidine kinase activity have been reported and scientific interest in this area is growing rapidly. This review presents a global overview of our current knowledge of the NME family and histidine phosphorylation, highlighting the underappreciated protein-histidine kinase activity of NME family members, specifically in human cells. In parallel, information about the structural and functional aspects of the NME family, and the knowns and unknowns of histidine kinase involvement in cell signaling are summarized.

The Function of NM23-H1/NME1 and Its Homologs in Major Processes Linked to Metastasis

Metastasis suppressor genes (MSGs) inhibit different biological processes during metastatic progression without globally influencing development of the primary tumor. The first MSG, NM23 (non-metastatic clone 23, isoform H1) or now called NME1 (stands for non-metastatic) was identified some decades ago. Since then, ten human NM23 paralogs forming two groups have been discovered. Group I NM23 genes encode enzymes with evolutionarily highly conserved nucleoside diphosphate kinase (NDPK) activity. In this review we summarize how results from NDPKs in model organisms converged on human NM23 studies. Next, we examine the role of NM23-H1 and its homologs within the metastatic cascade, e.g. cell migration and invasion, proliferation and apoptosis. NM23-H1 homologs are well known inhibitors of cell migration. Drosophila studies revealed that AWD, the fly counterpart of NM23-H1 is a negative regulator of cell motility by modulating endocytosis of chemotactic receptors on the surface of migrating cells in cooperation with Shibire/Dynamin; this mechanism has been recently confirmed by human studies. NM23-H1 inhibits proliferation of tumor cells by phosphorylating the MAPK scaffold, kinase suppressor of Ras (KSR), resulting in suppression of MAPK signalling. This mechanism was also observed with the C. elegans homolog, NDK-1, albeit with an inverse effect on MAPK activation. Both NM23-H1 and NDK-1 promote apoptotic cell death. In addition, NDK-1, NM23-H1 and their mouse counterpart NM23-M1 were shown to promote phagocytosis in an evolutionarily conserved manner. In summary, inhibition of cell migration and proliferation, alongside actions in apoptosis and phagocytosis are all mechanisms through which NM23-H1 acts against metastatic progression.

The 'known-knowns', and 'known-unknowns' of extracellular Nm23-H1/NDPK proteins

Nucleoside diphosphate kinases (NDPKs/NDK/NME) are a multifunctional class of proteins conserved throughout evolution. Whilst many of the functions of NDPKs have been identified as intracellular, extracellular eukaryotic and prokaryotic NDPK proteins are also detected in multiple systems and have been implicated in both normal physiology and disease. This review provides an overview of where the field stands on our developing understanding of how NDPK proteins get out of cells, the physiological role of extracellular NDPKs, and how extracellular NDPKs may signal to cells. We will also discuss some of the unanswered questions, the 'known-unknowns' that particularly warrant further investigation.

The NDPK/NME superfamily: state of the art

Nucleoside diphosphate kinases (NDPK) are nucleotide metabolism enzymes encoded by NME genes (also called NM23). Given the fact that not all NME-encoded proteins are catalytically active NDPKs and that NM23 generally refers to clinical studies on metastasis, we use here NME/NDPK to denote the proteins. Since their discovery in the 1950's, NMEs/NDPKs have been shown to be involved in multiple physiological and pathological cellular processes, but the molecular mechanisms have not been fully determined. Recent progress in elucidating these underlying mechanisms has been presented by experts in the field at the 10th International Congress on the NDPK/NME/AWD protein family in October 2016 in Dubrovnik, Croatia, and is summarized in review articles or original research in this and an upcoming issue of Laboratory Investigation. Within this editorial, we discuss three major cellular processes that involve members of the multi-functional NME/NDPK family: (i) cancer and metastasis dissemination, (ii) membrane remodeling and nucleotide channeling, and iii) protein histidine phosphorylation.

Correlation between NM23 protein overexpression and prognostic value and clinicopathologic features of ovarian cancer: a meta-analysis

OBJECTIVE

The prognostic value and clinicopathological features of NM23 (non-metastasis 23) have previously been assessed, but the results are controversial. Here, we attempted to clarify the correlation between NM23 expression and its prognostic value and the clinicopathological features in ovarian cancer (OC).

METHODS

The relevant studies were identified using PubMed, Embase, and Web of Science. We calculated the pooled odds ratio (OR) with 95% confidence intervals (CIs) for overall survival (OS), progression-free survival (PFS), and clinicopathological features. We used OS to evaluate the prognostic value of NM23 expression in patients with OC. Subgroup analyses were used to explore the source of heterogeneity.

RESULTS

We included 10 studies involving 894 patients in our assessment of the association between NM23 expression and OS for OC. Our data indicated that NM23 expression was not associated with improved OS (OR 0.83, 95% CI 0.41-1.68, P = 0.61) or PFS (OR 0.7, 95% CI 0.39-1.24, P = 0.22). Elevated NM23 expression was associated with differentiation grade (OR 0.35, 95% CI 0.2-0.6, P = 0.0002) and N status (OR 0.33, 95% CI 0.14-0.78, P = 0.01), whereas there was no significant difference between NM23 expression and tumor stage (OR 1.1, 95% CI 0.45-2.66, P = 0.84). Subgroup analysis did not reveal any potential source of heterogeneity. No obvious publication bias was found.

CONCLUSIONS

In OC, there is poor statistical significance between NM23 expression and OS and PFS, but NM23 expression is related to differentiation grade and N status. This meta-analysis reveals that NM23 expression is a potential factor of poor prognosis in OC. The prognostic role of NM23 in different OC stages in combination with the clinical characteristics suggests a novel approach for developing future therapeutic targets.

The role of NM23 in patients with colorectal cancer: A systematic review and meta-analysis

This meta-analysis was carried out to evaluate the relationship between NM23 expression and the prognosis of patients with colorectal cancer. We searched PubMed, EMBASE and Web of Science for relevant articles. The pooled odd ratios (ORs) and corresponding 95%CI were calculated to evaluate the prognostic value of NM23 expression in patients with colorectal cancer, and the association between NM23 expression and clinicopathological factors. In total, 2289 patients were pooled from 24 available studies. The incorporative OR combined by 16 studies with overall survival showed that high NM23 expression was associated with better overall survival (OR=0.67, 95%CI: 0.49-0.93, P=0.02, I ²=56%, Ph=0.004). And a new estimate without heterogeneity was produced when only combining high-quality studies (OR=0.70, 95%CI: 0.56-0.86, P=0.0007, I ²=46%). In disease free survival (DFS), we also obtained a good prognosis (OR=0.30, 95%CI: 0.14-0.68, P=0.004). Although we failed to find any significance in N status (P=0.10), elevated NM23 expression was related to well tumor differentiation (OR=0.60, 95%CI: 0.44-0.820, P=0.001) and Dukes' A&B (OR=0.55, 95%CI: 0.32-0.95, P=0.03). These results indicated that over-expressed NM23 might be an indicator of good prognosis, well tumor differentiation and Dukes' A&B of patients with colorectal cancer, but no significance was found in N status.

Metastasis suppressor NM23 limits oxidative stress in mammals by preventing activation of stress-activated protein kinases/JNKs through its nucleoside diphosphate kinase activity

NME1 (nonmetastatic expressed 1) gene, which encodes nucleoside diphosphate kinase (NDPK) A [also known as nonmetastatic clone 23 (NM23)-H1 in humans and NM23-M1 in mice], is a suppressor of metastasis, but several lines of evidence-mostly from plants-also implicate it in the regulation of the oxidative stress response. Here, our aim was to investigate the physiologic relevance of NDPK A with respect to the oxidative stress response in mammals and to study its molecular basis. NME1-knockout mice died sooner, suffered greater hepatocyte injury, and had lower superoxide dismutase activity than did wild-type (WT) mice in response to paraquat-induced acute oxidative stress. Deletion of NME1 reduced total NDPK activity and exacerbated activation of the stress-related MAPK, JNK, in the liver in response to paraquat. In a mouse transformed hepatocyte cell line and in primary cultures of normal human keratinocytes, MAPK activation in response to H₂O₂ and UVB, respectively, was dampened by expression of NM23-M1/NM23-H1, dependent on its NDPK catalytic activity. Furthermore, excess or depletion of NM23-M1/NM23-H1 NDPK activity did not affect the intracellular bulk concentration of nucleoside di- and triphosphates. NME1-deficient mouse embryo fibroblasts grew poorly in culture, were more sensitive to stress than WT fibroblasts, and did not immortalize, which suggested that they senesce earlier than do WT fibroblasts. Collectively, these results indicate that the NDPK activity of NM23-M1/NM23-H1 protects cells from acute oxidative stress by inhibiting activation of JNK in mammal models.-Peuchant, E., Bats, M.-L., Moranvillier, I., Lepoivre, M., Guitton, J., Wendum, D., Lacombe, M.-L., Moreau-Gaudry, F., Boissan, M., Dabernat, S. Metastasis suppressor NM23 limits oxidative stress in mammals by preventing activation of stress-activated protein kinases/JNKs through its nucleoside diphosphate kinase activity.

Prognostic and clinicopathological value of NM23 expression in patients with breast cancer: A systematic review and meta-analysis

It is hypothesized that, NM23, as a metastasis suppressor gene, may be a good indicator of patients with breast cancer in most reports. The aim of our meta-analysis was to determine the prognostic value of NM23 in patients with breast cancer synthetically, by searching 3 databases, PubMed, EMBASE, and Web of Science, for relevant articles. The inclusion criteria, exclusion criteria, and the standard-of-quality assessment were used according to a previous protocol. The pooled odd ratios (ORs) and corresponding 95% CI were calculated to assess the primary end point, survival data, and the secondary end point, associations between NM23 expression and clinicopathological factors. Finally, funnel plots and Egger׳s linear regression test were used to assess the potential publication bias. Overall, 792 articles were retrieved in the initial search of databases, and 4968 patients were eventually pooled from 26 available studies selected out by 2 independent reviewers. The incorporative OR showed that elevated NM23 expression was associated with better overall survival (OR = 0.62; 95% CI: 0.52-0.74; P < 0.00001; I² = 0%; Ph = 0.46). In disease-free survival, we also obtained a good prognosis (OR = 0.30; 95% CI: 0.18-0.48; P < 0.00001; I² = 46%; Ph = 0.13). In addition, high-NM23 expression was correlated with well or moderate histologic grade, negative lymph node metastasis, and early tumor staging. Furthermore, publication bias was detected in overall survival but not in disease-free survival, and it could also be verified by Egger׳s test (P = 0.009 and P = 0.687, respectively). These results implied that NM23 might be an indicator of good prognosis in patients with breast cancer, although further researches need to be performed to confirm the prognostic value of NM23.

The role of microRNA-196a in tumorigenesis, tumor progression, and prognosis

MicroRNAs are a large group of non-coding RNAs that have emerged as regulators of various biological processes, especially carcinogenesis and cancer progression. Recent evidence has shown that microRNA-196a (miR-196a) is upregulated in most types of tumors and involved in multiple biological processes via translational inhibition and mRNA cleavage, such as cell proliferation, migration, and invasion, mostly functioning as an oncogene. Dysregulation of miR-196a promotes oncogenesis and tumor progression. In this review, we summarize the upstream regulators, target genes, signaling pathways, and single nucleotide polymorphisms of miR-196a, which collectively affect cell proliferation, migration, and invasion. In addition, we review the clinical outcomes and significance of miR-196a. miR-196a may serve as a novel biomarker or target for diagnosis, prognosis, and therapy in several human cancers.

Evaluation of serum nucleoside diphosphate kinase A for the detection of colorectal cancer

We previously described the over-expression of nucleoside diphosphate kinase A (NDKA) in tumours and serum from colorectal cancer (CRC) patients, suggesting its use as biomarker. In this study we evaluated the diagnostic accuracy of serum NDKA to detect advanced neoplasia (CRC or advanced adenomas). Furthermore, the performance of NDKA was compared with the faecal immunochemical test (FIT). The study population included a case-control cohort and a screening cohort (511 asymptomatic first-degree relatives of CRC patients that underwent a colonoscopy and a FIT). Serum NDKA was elevated in CRC patients in the case-control cohort (p = 0.002). In the screening cohort, NDKA levels were higher for advanced adenomas (p = 0.010) and advanced neoplasia (p = 0.006) compared to no neoplasia. Moreover, elevated NDKA was associated with severe characteristics of adenomas (≥3 lesions, size ≥ 1 cm or villous component). Setting specificity to 85%, NDKA showed a sensitivity of 30.19% and 29.82% for advanced adenomas and advanced neoplasia, respectively. NDKA combined with FIT (100 ng/mL cut-off) detected advanced adenomas and advanced neoplasia with 45.28% and 49.12% sensitivity, with specificity close to 90%. The combination of serum NDKA and FIT can improve the detection of advanced neoplasia, mainly for lesions located on the proximal colon, in asymptomatic individuals with CRC family-risk.

Sponge non-metastatic Group I Nme gene/protein - structure and function is conserved from sponges to humans

Background

Nucleoside diphosphate kinases NDPK are evolutionarily conserved enzymes present in Bacteria, Archaea and Eukarya, with human Nme1 the most studied representative of the family and the first identified metastasis suppressor. Sponges (Porifera) are simple metazoans without tissues, closest to the common ancestor of all animals. They changed little during evolution and probably provide the best insight into the metazoan ancestor's genomic features. Recent studies show that sponges have a wide repertoire of genes many of which are involved in diseases in more complex metazoans. The original function of those genes and the way it has evolved in the animal lineage is largely unknown. Here we report new results on the metastasis suppressor gene/protein homolog from the marine sponge Suberites domuncula, NmeGp1Sd. The purpose of this study was to investigate the properties of the sponge Group I Nme gene and protein, and compare it to its human homolog in order to elucidate the evolution of the structure and function of Nme.

Results

We found that sponge genes coding for Group I Nme protein are intron-rich. Furthermore, we discovered that the sponge NmeGp1Sd protein has a similar level of kinase activity as its human homolog Nme1, does not cleave negatively supercoiled DNA and shows nonspecific DNA-binding activity. The sponge NmeGp1Sd forms a hexamer, like human Nme1, and all other eukaryotic Nme proteins. NmeGp1Sd interacts with human Nme1 in human cells and exhibits the same subcellular localization. Stable clones expressing sponge NmeGp1Sd inhibited the migratory potential of CAL 27 cells, as already reported for human Nme1, which suggests that Nme's function in migratory processes was engaged long before the composition of true tissues.

Conclusions

This study suggests that the ancestor of all animals possessed a NmeGp1 protein with properties and functions similar to evolutionarily recent versions of the protein, even before the appearance of true tissues and the origin of tumors and metastasis.

Nm23-H1 suppresses hepatocarcinoma cell adhesion and migration on fibronectin by modulating glycosylation of integrin beta1

Background

Nm23 gene was isolated as a metastatic suppressor gene. The antimetastatic effect of Nm23 has been an enigma for more than 10 years. Little is known about its molecular mechanisms. In this study we overexpressed Nm23-H1 in H7721 cells and observed reduction of cell adhesion, migration and extension of actin stress fibers in cells stimulated by fibronectin (Fn).

Methods

pcDNA3/Nm23-H1 was introduced into H7721 cells, and expression of Nm23-H1 was monitored by RT-PCR and western blot. Cell adhesion, actin extension and wound-induced migration assays were done on dishes coated with fibronectin. Phosphorylation of focal adhesion kinase (FAK) and total amount of integrin alpha5 and beta1 in Nm23-H1 transfected cells and control cells were measured by western blot. Flow cytometry was used to detect expression of surface alpha5 and beta1 integrin. N-glycosylation inhibitor tunicamycin was used to deglycosylate the integrin beta1 subunit.

Results

Overexpression of nm23-H1 in H7721 cells reduced cell adhesion, migration and extension of actin stress fibers on dishes coated with Fn. Phosphorylation of FAK in Nm23-H1 transfected cells was also attenuated. Integrin alpha5 and beta1 gene messages were unaltered in nm23-H1 overexpressed cells as detected by RT-PCR. However, while cell surface integrin alpha5 was unchanged, surface expression of beta1 integrin was downregulated. Western blot also showed that the total amounts of integrin alpha5 and beta1 were unaltered, but the level of mature integrin beta1 isoform was decreased significantly. Furthermore, partially glycosylated precursor beta1 was increased, which indicated that the impaired glycosylation of integrin beta1 precursor might contribute to the loss of cell surface integrin beta1 in nm23-H1 overexpressed cells.

Conclusion

These results suggest that by modulating glycosylation of integrin beta1, nm23-H1 down-regulates integrin beta1 subunit on cell surface and mediates intracellular signaling and subsequent suppression of the invasive process, including cell adhesion and migration.

Significance of DR-nm23 protein expression in colorectal carcinoma

AIM: To investigate the relationship between the expression of DR-nm23 protein and the carcinogenesis, progression and metastasis of colorectal carcinoma.

METHODS: Ninety-eight colorectal carcinoma specimens, 57 adenoma specimens and 42 normal colorectal tissue specimens were examined by immunohistochemistry using the streptavidin-peroxidase method. The correlation of DR-nm23 protein expression with the carcinogenesis, progression, lymph node metastasis, histological type and differentiation grade of colorectal carcinoma was then analyzed.

RESULTS: The positive rate of DR-nm23 protein expression was significantly higher in normal colorectal tissue than in adenoma and colorectal carcinoma (71.4% vs 38.6% and 35.7%, respectively; both P < 0.01). The positive rate of DR-nm23 protein expression was significantly lower in high-grade intraepithelial tumors than in low-grade intraepithelial tumors in the adenoma group (25.7% vs 59.1%, P < 0.05), and in metastatic colorectal carcinoma than in non-metastatic colorectal carcinoma (23.1% vs 44.1%, P < 0.05). The expression of DR-nm23 is negatively related to lymph node metastasis (P < 0.05). DR-nm23 expression is also closely related to histological type (χ² = 13.731, P < 0.01) and differentiation grade (χ² = 12.198, P < 0.01). The positive rate of DR-nm23 protein was higher in secondary tumors than in primary tumors in the metastatic colorectal carcinoma group though no significant difference was noted between them (P > 0.05).

CONCLUSION: Decreased DR-nm23 protein expression is closely related to tumor differentiation, carcinogenesis, progression and metastasis in colorectal carcinoma. DR-nm23 is an important parameter for evaluation of the biological behavior and prognosis of colorectal carcinoma.

Regulation of transforming growth factor-beta-dependent cyclooxygenase-2 expression in fibroblasts

Abnormal transforming growth factor-beta (TGF-beta) signaling is a critical contributor to the pathogenesis of various human diseases ranging from tissue fibrosis to tumor formation. Excessive TGF-beta signaling stimulates fibrotic responses. Recent research has focused in the main on the antiproliferative effects of TGF-beta in fibroblasts, and it is presently understood that TGF-beta-stimulated cyclooxygenase-2 (COX-2) induction in fibroblasts is essential for antifibroproliferative effects of TGF-beta. Both TGF-beta and COX-2 have been implicated in tumor growth, invasion, and metastasis, and therefore tumor-associated fibroblasts are a recent topic of interest. Here we report the identification of positive and negative regulatory factors of COX-2 expression induced by TGF-beta as determined using proteomic approaches. We show that TGF-beta coordinately up-regulates three factors, heterogeneous nuclear ribonucleoprotein A/B (HNRPAB), nucleotide diphosphate kinase A (NDPK A), and nucleotide diphosphate kinase A (NDPK B). Functional pathway analysis showed that HNRPAB augments mRNA and protein levels of COX-2 and subsequent prostaglandin E(2) (PGE(2)) production by suppressing degradation of COX-2 mRNA. In contrast, NDPK A and NDPK B attenuated mRNA and protein levels of COX-2 by affecting TGF-beta-Smad2/3/4 signaling at the receptor level. Collectively, we report on a new regulatory pathway of TGF-beta in controlling expression of COX-2 in fibroblasts, which advances our understanding of pathophysiological mechanisms of TGF-beta.

Nm23-H1 promotes adhesion of CAL 27 cells in vitro

nm23-H1 was found to diminish metastatic potential of carcinoma cell lines and therefore was placed in the group of metastatic suppressor genes. Its protein product has a function of a nucleoside diphosphate kinase (NDPK) as well as protein kinase and nuclease. Though it was found that Nm23-H1 is involved in many cellular processes, it is still not known how it promotes metastatic suppressor activity. Since the process of metastasis is dependent on adhesion properties of cells, the goal of our work was to describe the adhesion properties of CAL 27 cells (oral squamous cell carcinoma of the tongue) overexpressing FLAG/nm23-H1. In our experiments, cells overexpressing nm23-H1 show reduced migratory and invasive potential. Additionally, cells overexpressing nm23-H1 adhere stronger on substrates (collagen IV and fibronectin) and show more spread morphology than the control cells. Results obtained by EGF induction of migration revealed that the adhesion strength predetermined cell response to chemoattractant and that Nm23-H1, in this cell type, does not interfere with, EGF induced, Ras signaling pathway. These data contribute to the overall knowledge about nm23-H1 and its role in cell adhesion, migration, and invasion, especially in oral squamous cell carcinoma.

Reversibility of nucleoside diphosphate kinase solubilization from the surface of the outer mitochondrial membrane

It was found that in medium with low ionic strength nucleoside diphosphate kinase (NDPK) solubilization from the outer membrane of liver mitochondria could be partially reversed by the addition of 3.3 mM MgCl2. Complete rebinding of the enzyme after the addition of MgCl2 was observed when the mitochondrial washing and storage medium contained leupeptin, an inhibitor of cathepsins. It was demonstrated that leupeptin and another inhibitor of cysteine proteinases, E-64, do not influence the rate of NDPK solubilization as well as its solubilized and membrane-associated activity. We conclude that NDPK becomes sensitive to proteolysis only after its solubilization; proteolysis does not affect the part of the enzyme molecule that is responsible for catalysis. After solubilization of NDPK in the absence of leupeptin, cathepsins damage sites of its binding on the membranes. The rate of the enzyme solubilization is dependent on the pH of the storage medium (pH 6.0-8.0); it decreases with increase in pH. It was shown that in the medium with high ionic strength, MgCl2 does not reverse pH-dependent NDPK solubilization, but solubilization could be reversed by increase in medium pH in the presence of E-64 and BSA. The physiological importance of these results is discussed.