Liposome-cell interactions: In vitro discrimination of uptake mechanism and in vivo targeting strategies to mononuclear phagocytes

The interactions of liposomes with cells have been extensively studied to determine their potential use as vehicles for the delivery of drugs in vivo. Since intravenously administered liposomes are, for the most part, cleared by cells of the reticuloendothelial system (RES), considerable effort has been made to take advantage of this phenomenon rather than view it as an obstacle. Indeed, cells of the RES, in particular macrophages, have been shown to play a vital role in homeostasis and in host defence mechanisms against infection and neoplasia.

In this article, we present an overview of liposome-cell interactions, with particular emphasis on the techniques used to monitor the interaction of liposomes with macrophages. Specifically, we discuss methodologies which can be used to differentiate between liposome-cell fusion, adsorption and endocytosis in vitro. In addition, we outline the various strategies that have been employed for both actively and passively targeting liposomes to macrophages in vivo. We also review the rationale and various techniques for designing liposomes for enhanced macrophage uptake, which, in certain cases, results in the selective release of liposome-entrapped compounds in situ.     

Differences in antibody-dependent cellular cytotoxicity against tumor cells in in vitro differentiating mononuclear phagocytes from bone marrows of normal and inflamed mice

Mononuclear phagocytes, differentiated in vitro from bone marrow cells of mice inflamed in vivo with either Corynebacterium parvum or thioglycollate, expressed a higher activity in antibody-dependent cellular cytotoxicity (ADCC) against tumor cells, than those of normal mice. A good correlation between the cytolytic activity and chemiluminescence activity of the different mononuclear phagocyte populations was observed. The ADCC activity of BMDMP from normal mice was inhibited by exogenous prostaglandin E₂ (PGE₂) to a higher extent than that of BMDMP of inflamed mice. When the three BMDMP populations were cultured in the presence of aspirin (without exogenously added PGE₂), the ADCC was significantly increased. The three populations gave identical high values. This suggests that the differential ADCC activity of BMDMP from normal and inflamed mice is due to their differential response to endogenous prostaglandins. PGE₂ showed also a differential effect on the mononuclear phagocyte-forming capacity of bone marrow macrophage precursor cells from normal and inflamed mice. Bone marrow precursor cells from inflamed mice showed a higher resistance to the suppressive effect of PGE₂ (10^?5M) on mononuclear phagocyte-forming capacity than those of normal mice which were totally suppressed. It is concluded that the observed differential properties of the three bone marrow-derived mononuclear phagocyte populations originate at the level of bone marrow precursor cells and that, therefore, the similar functional differences observed in inflammation-induced peritoneal macrophage populations, observed by our and other groups, stem at least partly from differences at the level of bone marrow precursor cells.     

Cyclic AMP response to prostaglandin E1 in mononuclear cells from peripheral blood and synovial fluid of patients with rheumatoid arthritis.

The cyclic AMP response to prostaglandin E1 (PGE1) was studied in peripheral blood (PB) and synovial fluid (SF) mononuclear cells from patients with rheumatoid arthritis (RA). The PGE1 induced accumulation of cyclic AMP was consistently (7 of 8 patients) less in cell suspensions derived from SF than in suspensions of equivalent numbers of mononuclear cells obtained simultaneously from PB. The high PB/SF cyclic AMP ratio was seen most clearly at the lowest concentration (10(-6)M) of PGE1 tested. There was no correlation between the patients' therapy and cyclic AMP response to PGE1. The high PB/SF cyclic AMP ratio was not accounted for by the presence of platelets in PB cell suspensions.     

P2X7 receptor-mediated scavenger activity of mononuclear phagocytes toward non-opsonized particles and apoptotic cells is inhibited by serum glycoproteins but remains active in cerebrospinal fluid

Rapid phagocytosis of non-opsonized particles including apoptotic cells is an important process that involves direct recognition of the target by multiple scavenger receptors including P2X7 on the phagocyte surface. Using a real-time phagocytosis assay, we studied the effect of serum proteins on this phagocytic process. Inclusion of 1-5% serum completely abolished phagocytosis of non-opsonized YG beads by human monocytes. Inhibition was reversed by pretreatment of serum with 1-10 mM tetraethylenepentamine, a copper/zinc chelator. Inhibitory proteins from the serum were determined as negatively charged glycoproteins (pI < 6) with molecular masses between 100 and 300 kDa. A glycoprotein-rich inhibitory fraction of serum not only abolished YG bead uptake but also inhibited phagocytosis of apoptotic lymphocytes or neuronal cells by human monocyte-derived macrophages. Three copper- and/or zinc-containing serum glycoproteins, ceruloplasmin, serum amyloid P-component, and amyloid precursor protein, were identified, and the purified proteins were shown to inhibit the phagocytosis of beads by monocytes as well as phagocytosis of apoptotic neuronal cells by macrophages. Human adult cerebrospinal fluid, which contains very little glycoprotein, had no inhibitory effect on phagocytosis of either beads or apoptotic cells. These data suggest for the first time that metal-interacting glycoproteins present within serum are able to inhibit the scavenger activity of mononuclear phagocytes toward insoluble debris and apoptotic cells.     

Effects of products from macrophages, blood mononuclear cells and of retinol on collagenase secretion and collagen synthesis in chondrocyte culture

Collagenase secretion was studied in cultures of rabbit articular chondrocytes. Differentiation of the cells was assessed by characterizing the type of ³H-labelled collagen produced during treatment with (1) conditioned media from rabbit peritoneal macrophages and human blood mononuclear cells, and (2) with retinol, a potent cartilage resorbing agent in tissue culture. Conditioned media stimulated collagenase secretion. Total collagen synthesis was reduced due to a decrease of synthesis of α₁ chains; the amount of α₂ chains synthesized was unchanged. This is thought to be due to a reduction in type II synthesis. Retinol did not stimulate collagenase secretion. Total collagen synthesis was reduced by retinol. α₂ chain synthesis, however, was significantly increased, suggesting a switch of collagen synthesis in favor of type I collagen and, therefore, dedifferentiation. These results demonstrate that dedifferentiation of chondrocytes with respect to collagen synthesis is not necessarily associated with a stimulation of collagenase secretion.     

Cyclic AMP response to prostaglandin E1 in mononuclear cells from peripheral blood and synovial fluid of patients with rheumatoid arthritis

The cyclic AMP response to prostaglandin E₁ (PGE₁) was studied in peripheral blood (PB) and synovial fluid (SF) mononuclear cells from patients with rheumatoid arthritis (RA). The PGE₁ induced accumulation of cyclic AMP was consistently (7 of 8 patients) less in cell suspensions derived from SF than in suspensions of equivalent numbers of mononuclear cells obtained simultaneously from PB. The high PB/SF cyclic AMP ratio was seen most clearly at the lowest concentration (10⁻⁶M) of PGE₁ tested. There was no correlation between the patients' therapy and cyclic AMP response to PGE₁. The high PB/SF cyclic AMP ratio was not accounted for by the presence of platelets in PB cell suspensions.     

Requirements for different Ca2+ pools in the activation of rabbit platelets: I. Release reaction and protein phosphorylation

We examined the role of Ca²⁺, both extracellular and intracellular in origin, in the release reaction and protein phosphorylation in rabbit platelets stimulated with platelet activating factor (acetylglyceryl ether phosphorylcholine), thrombin, or ionophore A23187. In the presence of extracellular Ca²⁺, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8), a putative antagonist of intracellular Ca²⁺ transport, blocked platelet activating factor-initiated serotonin release at a half-maximal inhibitor concentration of 40 μM, compared to 350 μM for thrombin-induced release and greater than 500 μM, for A23187-induced release. Platelet activating factor-induced phosphorylation of two platelet proteins of M_r=41 000 (P7P) and 20 000 (P9P) was inhibited by TMB-8, an effect which was additive to that caused by removing extracellular Ca²⁺. TMB-8 demonstrated only minor to non-existant inhibitory effects on phosphorylation in thrombin- or A23187-stimulated platelets. In contrast to P9P phosphorylation, phosphorylation of P7P caused by platelet activating factor was more dependent on a TMB-8 sensitive step than on the availability of extracellular Ca²⁺. Experiments with buffers containing fixed concentrations of free Ca²⁺ revealed that both processes (release and phosphorylation), when stimulated by platelet activating factor and thrombin, had the same threshold requirement (1–3 μM) for extracellular free Ca²⁺. These studies provide evidence that stimulation of rabbit platelets by platelet activating factor is more dependent on a TMB-8-sensitive intracellular Ca²⁺ source than is stimulation caused by thrombin. Furthermore, our data indicate that activation of different intracellular processes involved in platelet secretion (such as P7P and P9P phosphorylation) may require Ca²⁺ from different pools.     

Release of IL-4 and IL-5 from human peripheral blood mononuclear cells co-cultured with Japanese cedar pollen antigen in vitro

The Japanese cedar pollen (JCP) is a major allergen with respect to pollinosis in Japan. It is believed that interleukin-4 (IL-4) and interleukin-5 (IL-5) derived from lymphocytes and other cells play a pivotal role in allergic reactions. We investigated whether the JCP antigen stimulates the release of these cytokines by peripheral blood mononuclear cells (PBMCs). PBMCs from eight adults (five adults with JCP pollinosis and three adults without JCP pollinosis) were co-incubated with purified JCP antigens. IL-4 was released in response to JCP antigens in six of the eight subjects at 24 h and in three subjects at 48 h. IL-4 release at 24 h occurred in all five subjects with JCP pollinosis but in only one of the three subjects without pollinosis. IL-5 was released in response to the JCP antigen in five of the eight subjects at 24 h and 48 h, including four of the five subjects with JCP pollinosis and one of the three subjects without pollinosis. These results suggest that PBMCs were more likely to release IL-4 and IL-5 in the presence of JCP pollinosis.     

Specific activation of cyclic AMP-dependent protein kinase(s) by H2-histamine agonists in isolated gastric mucosal cells from guinea-pig

Histamine stimulated cyclic AMP-dependent protein kinase activity in dispersed mucosal cells from guinea-pig gastric fundus (K_a = 5 μM). The H₂-agonists dimaprit and impromidine produced similar effects, while the H₁-agonist 2-(2-pyridyl) ethylamine had only a weak one. The H₂-antagonist cimetidine competitively inhibited 0.1 mM histamine stimulation (K_i = 2 μM). In contrast, the H₁-antagonist diphenhydramine had no effect up to 1 mM.     

Metabolic and secretory response of tumoral-insulin producing cells to D-fructose and D-galactose

At variance with normal islet cells, tumoral insulin-producing cells of the RINm5F line were found to display a positive secretory response not solely to D-glucose and D-mannose, but also to D-fructose and D-galactose. All hexoses increased the ATP/ADP ratio, exerted a sparing action upon the oxidation of endogenous nutrients in cells prelabelled with either L-[U-¹⁴C]glutamine or [U-¹⁴C]palmitate, increased the output of lactic acid and, as judged from data collected in the presence of D-[U-¹⁴C]hexoses, underwent oxidation in the RINm5F cells. The secretory response to these four hexoses appeared commensurate with the extent of their metabolic effects.     

Release of VIP-like immunoreactivity in response to dopamine agonists, insulin hypoglycemia and feeding in conscious dogs

VIP levels were measured by radioimmunoassay in peripheral venous blood of conscious dogs. Bolus injections of the dopamine agonists apomorphine, 0.05 mg/kg, and bromocriptine, 0.2 mg/kg, were found to increase VIP levels from approximately 5 pmol/l to 150 pmol/l. The release responses were abolished by pretreatment with dopamine antagonists (haloperidol 0.1 mg/kg or halopemid 0.1 mg/kg) and by hexamethonium (1 mg/kg) a blocker of ganglionic transmission. Vagotomy did not inhibit the dopamine agonist induced output of VIP. Vagal activation by means of feeding or insulin hypoglycemia caused only minor rises of VIP levels (5-10 pmol/l). It is concluded that dopamine agonists stimulate the release of VIP from populations of neurons other than those affected by vagal and sympathetic activation. Possible sites of action for the VIP releasing effect exerted by dopamine agonists are discussed. Furthermore, it is suggested that some of the peripheral effects exerted by dopaminergic drugs are exerted via a previous release of VIP.     

The effects of monocyte-conditioned medium and interleukin 1 on the synthesis of collagenous and non-collagenous proteins by mouse bone and human bone cells in vitro

Cultural adherent human mononuclear cells produce factor(s) which stimulate the release of calcium from new-born mouse calvaria in organ culture. This stimulation of bone resorption is accompanied by an inhibition of the incorporation of [³H]proline into collagen which is independent of increased prostaglandin production by the bone. When human osteoblast-like cells are treated with conditioned medium from human mononuclear cells, collagen accounts for a decreased proportion of the protein synthesised. This effect on matrix synthesis is not accompanied by an inhibitory action of the monocyte-conditioned medium preparations on net cell proliferation. In human osteoblast-like cell cultures, partially purified human interleukin 1 also inhibits the production of the bone-specific protein osteocalcin in a dose-dependent fashion. These observations are consistent with the hypothesis that products of human monocytes similar to, or identical with, human interleukin 1 may be important regulators of bone metabolism and may contribute to the bone loss seen in diseases such as chronic rheumatoid arthritis.     

Effects of Arachidonic Acid on Dopamine Synthesis, Spontaneous Release, and Uptake in Striatal Synaptosomes from the Rat

Abstract: Arachidonic acid (AA) markedly stimulated, in a dose-dependent manner, the spontaneous release of [³H]dopamine ([³H]DA) continuously synthesized from [³H]tyrosine in purified synaptosomes from the rat striatum. As estimated by simultaneous measurement of the rate of [³H]H₂O formation (an index of [³H]tyrosine conversion into [³H]DOPA), the AA response was associated with a progressive and dose-dependent reduction of [³H]DA synthesis. In contrast to AA, arachidic acid, oleic acid, and the methyl ester of AA (all at 10⁻⁴ M ) did not modify [³H]DA release. The AA (3 × 10⁻⁵ M )-evoked release of [³H]DA was not affected by inhibiting AA metabolism, with either 5,8,11,14-eicosatetraynoic acid or metyrapone, suggesting that AA acts directly and not through one of its metabolites. AA also inhibited in a dose-dependent manner [³H]DA uptake into synaptosomes, with a complete blockade observed at 10⁻⁴ M . However, AA (10⁻⁴ M ) still stimulated [³H]DA spontaneous release in the presence of either nomifensine or other DA uptake inhibitors, indicating that AA both inhibits DA reuptake and facilitates its release process. Finally, the AA (10⁻⁴ M )-evoked release of [³H]DA was not affected by protein kinase A inhibitors (H-89 or Rp -8-Br-cAMPS) but was markedly reduced in the presence of protein kinase C inhibitors (Ro 31-7549 or chelerythrine).     

Rapid purification and activity of apolipoprotein CI on the proliferation of bovine vascular endothelial cells in vitro

The growth-promoting activity of human high-density lipoproteins (HDL) and of their apolipoprotein components on bovine vascular endothelial cells in vitro has been compared. When maintained on plastic culture dishes and exposed to medium containing lipoprotein-deficient serum and fibroblast growth factor, these cells do not proliferate. Addition of either HDL or the total HDL apolipoproteins induces significant cell proliferation. Apolipoprotein C_I, purified by chromatography on the ion-exchanger resin Polybuffer exchanger 94, has an effect on the cell growth similar to that of the total apolipoproteins of HDL.     

Translocation and Activation of Protein Kinase C in Striatal Neurons in Primary Culture: Relationship to Phorbol Dibutyrate Actions on the Inositol Phosphate Generating System and Neurotransmitter Release

The actions of the tumor-promoting phorbol ester phorbol dibutyrate were examined, under identical physiological conditions, on three distinct cellular processes in striatal neurons: the distribution of protein kinase C, the carbachol-stimulated generation of [3H]inositol monophosphate, and the KCl-evoked release of gamma-[3H]aminobutyric acid ([3H]GABA). Phorbol dibutyrate induced a rapid (complete in 5 min), dose-dependent, entirely reversible (t0.5 = 15 min) translocation of protein kinase C from cytosol to membrane. On longer exposure to phorbol dibutyrate, membrane-associated protein kinase C returned toward the control level, and total cellular enzyme activity declined markedly. Phorbol dibutyrate also induced the dose-dependent attenuation of carbachol-stimulated [3H]inositol monophosphate production and potentiation of KCl-evoked release of [3H]GABA. The translocation of protein kinase C and the potentiation of KCl-evoked [3H]GABA release were both rapidly reversed following washout of phorbol dibutyrate. In addition, for both processes, the effect of a 1-h exposure to phorbol dibutyrate was markedly less than that observed following a 5-min exposure to the agent. In direct contrast, inhibition of carbachol-stimulated [3H]inositol monophosphate production was not rapidly reversed following washout of phorbol dibutyrate and was actually more pronounced following a 1-h exposure, compared with a 5-min exposure. These findings indicate that some, but not all, of the actions of phorbol dibutyrate are closely associated with the translocation of protein kinase C in striatal neurons in primary culture.     

Growth and differentiation of murine cartilage cells in vitro followng a short-term exposure to triamcinolone acetonide

Summary Measurements of ³H-thymidine incorporation, quantitative autoradiography and morphometry were used to evaluate cell behavior during the recovery of mandibular condylar cartilage cultures following short-term exposure to a corticosteroid hormone in vitro. Apical segments of mandibular condyles of newborn mice were initially incubated in the presence of the hormone triamcinolone acetonide (10^-6 M) for 24 h and were thereafter cultured for additional 6 days in hormone-free medium. The present results indicated that the treatment led to a decrease in the rate of incorporation of ³H-thymidine, a feature that lasted for 48 h following the removal of the hormone. Quantitative ³H-thymidine autoradiography of explants that were labeled in the presence of the hormone further substantiated the initial suppressive effect of the hormone on cellular proliferation, a feature that was followed by a recovery. Differences were noted in the pattern of distribution of labeled cells: in control explants, labeled cells progressively moved from the chondroprogenitor compartment into the differentiated portion of the cartilage; in hormone-treated explants, ³H-thymidine labeled cells were confined to the progenitor layer up to 5 days after the treatment and only then appeared in the chondrocytic compartment. The hormone's adverse effect upon differentiation was manifested by both morphology, and by causing a significant increase in the size of the progenitor layer (up to 50.5% on 4th post-treatment day) along with a 70.5% reduction in the size of chondroblastic layer. We conclude that a short-term exposure to a glucocorticoid hormone in vitro interferes with proliferation of chondroprogenitor cells and their subsequent differentiative pathway. While the proliferative activity was restored within 48 h, the hormone's effect on differentiation lasted for a considerably longer period of time.     

Identification of products of protein phosphorylation in T37-transformed cells and comparison with normal cells

Proteins from crown gall tissue labelled in vivo with [³²P]orthophosphate were analysed by SDS-polyacrylamide gel electrophoresis. The major phosphorylated proteins were of 50.6 and 48.3 kDa, with minor bands at 80.1, 73.9, 68, 40.4, 30, 21.5, 20.2 and 15.2 kDa. Partial hydrolysates of total ³²P-labelled proteins were analysed in a number of ways. A two-dimensional separation on paper by electrophoresis in pyridine/acetic acid at pH 3.5 followed by chromatography in isobutyric acid/0.5 M ammonia revealed radioactive spots coincident with phosphoserine and phosphothreonine markers and only partially coincident with the phosphotyrosine marker. Two-dimensional electrophoresis at pH 1.9 followed by pH 3.5, however, unequivocally showed the presence of phosphotyrosine after elution of the phosphotyrosine marker. Phosphoserine, phosphothreonine and phosphotyrosine were present in the ratio 89.4:8.5:2.1. This is a much higher level of phosphotyrosine than normally found in animal cells. The three phosphoamino acids were confirmed by chromatography with authentic samples in four solvent systems on cellulose or silica TLC, and by dansylation followed by silica TLC. The radioactive compound running almost coincident with phosphotyrosine on two-way electrophoresis, pH 3.5, followed by chromatography in isobutyric acid/0.5 M ammonia was identified tentatively as uridine 5′-monophosphate on the basis of electrophoretic and chromatographic behaviour. Further experiments to compare normal (growing and non-growing) tobacco callus and T37-transformed cells did not give markedly different ratios of the three phosphoamino acids, although the rapidly-growing normal tobacco (i.e., plus cytokinin) appeared to have a greater abundance of the two minor phosphoamino acids (approx. 2-times). The lack of effect of transformation is in contrast to animal cells where transformation results in a 10-fold increase in the virally affected cells.     

Enhanced proinflammatory response of mononuclear cells to in vitro LPS-challenge in patients with ventricular fibrillation in the setting of acute myocardial infarction

Aims. Ventricular fibrillation (VF) in the setting of acute myocardial infarction (AMI) is the leading cause of sudden cardiac death. A potential role of intrinsic, subclinical inflammatory states in patients suffering from ischemia-related VF has not been investigated yet. The aim of the present study was (i) to examine serum levels of proinflammatory markers in VF survivors and (ii) to evaluate basal and lipopolysaccharide (LPS)-stimulated interleukin-8-mRNA (IL-8-mRNA) levels in patients with and without VF complicating AMI. Methods. Twenty-five patients with a history of VF during AMI and a control group of 25 AMI patients without VF were included. Blood samples were taken remote from AMI with a mean of 590 days. Circulating serum levels of IL-8, IL-6, soluble E-selectin (sE-selectin), tissue factor activity (TFA), tissue inhibitor of matrix-metalloproteinase-1 (TIMP-1) and matrix-metalloproteinase-9 (MMP-9) were measured. Mononuclear cells were isolated by density gradient centrifugation. The cells were stimulated with lipopolysaccharide (LPS) from Escherichia coli (700 ng/mL). IL-8-mRNA levels in mononuclear cells were determined by a colorimetric mRNA quantification assay. Results. Serum levels (median; range) of IL-8 (VF: 2.24 pg/mL; <0.10–19.3 pg/mL versus controls: 0.10 pg/mL; <0.10–7.7 pg/mL; p = 0.014), IL-6 (VF: 0.68 pg/mL; <0.05–2.9 pg/mL versus controls: 0.23 pg/mL; <0.05–1.8 pg/mL; p = 0.042) and TIMP-1 (VF: 229 ng/mL; 144–348 ng/mL versus controls: 186 ng/mL; 126–263 ng/mL; p = 0.014) were significantly higher among patients with VF as compared to controls. Baseline IL-8-mRNA concentrations of blood mononuclear cells were significantly higher among patients with VF (257 amol/mL; 52–2672 amol/mL) as compared to patients without VF (37 amol/mL, 3.2–770 amol/mL; p < 0.01). IL-8-mRNA levels after LPS-challenge were significantly higher among patients with VF (3503 amol/mL; 215–13,573 amol/mL) than in patients without VF (1003 amol/mL; 208–3386 amol/mL; p < 0.01). Conclusions. Circulating IL-8, IL-6, and TIMP-1 concentrations as well as IL-8-mRNA expression in mononuclear cells at baseline and after LPS-challenge are increased among patients with a history of VF in the setting of AMI as compared to patients without VF. These findings indicate an enhanced inflammatory response to a proinflammatory stimulus in VF survivors. The magnitude of this increased acute phase reactants may indicate a novel pathway of arrhythmogenesis in patients with AMI.     

Calcium-dependent proteins in platelets response of calcium-activated protease in normal and thrombasthenic platelets to aggregating agents

Recent studies have suggested a role for Ca²⁺-dependent proteolysis in the regulation of microfilament disassembly by high molecular weight actin-binding protein. A Ca²⁺-activated protease similar to myofibrillar Ca²⁺-activated protease has been described in platelets. To explore the role of Ca²⁺-activated proteolysis of actin-binding protein in platelet function, we have examined the effects of platelet aggregating agents on platelet Ca²⁺-activated protease-like activity. The hydrolysis of actin-binding protein by Ca²⁺-activated protease was determined electrophoretically. The calcium ionophore, A23187, produced a dose-dependent stimulation of Ca²⁺-activated protease-like activity in the presence of exogenous calcium but had no effect in the absence of external calcium. Both normal and thrombasthenic platelets generated Ca²⁺-activated protease-like activity in response to A23187. Ionophore-induced stimulation of Ca²⁺-activated protease-like activity was not affected by prior incubation of platelets with 8-bromo cyclic GMP, 8-bromo cyclic AMP, prostaglandin E₁, prostaglandin I₂, indomethacin or tetracaine, but was inhibited by the sulfhydryl inhibitor N-ethylmaleimide. These results confirm the presence of Ca²⁺-activated protease in platelets and indicate that the source of calcium important in Ca²⁺-activated protease stimulation is in part extracellular. Other aggregating agents, thrombin, epinephrine, and ADP, were not accompanied by hydrolysis of actin-binding protein, indicating that the alteration in ionic calcium that occurs during aggregation by these other agents is insufficient to generate Ca²⁺-activated protease-like activity as measured by the present analytical technique.