Solubilization of glucagon and epinephrine sensitive adenylate cyclase from rat liver plasma membranes

Hormonally sensitive adenylate cyclase has been solubilized from rat liver plasma membranes using Triton X-305 in Tris buffers containing mercaptoethanol and MgCl₂. The solubilized enzyme was stimulated 5 fold by NaF, 7 fold by glucagon and 20 fold by epinephrine. Criteria for solubilization included lack of sedimentation at 100,000 × g for one hour, the absence of particulate material in the 100,000 × g supernatant when examined by electron microscopy, and inclusion of hormonally sensitive adenylate cyclase activity in Sephadex G 200 gels. The molecular weight of the solubilized, hormonally sensitive enzyme was approximately 200,000 in the presence of Triton X-305.     

Regulation of intracellular cyclic AMP levels in the white-rot fungus Phanerochaete chrysosporium during the onset of idiophasic metabolism

Adenylate cyclase activity in Phanerochaete chrysosporium was present in cell fractions sedimenting at 1,000xg, 15,000xg, and in the 150,000xg supernatant. A small amount of activity in the 1,000xg pellet could be solubilised by treatment with Triton X-100, and the enzyme in all fractions required an ATP-Mn²⁺ substrate. Adenylate cyclase activity in the 150,000xg pellet was low (0.003 nmol/mg protein·min) and may have resulted from contamination by other fractions. Highest adenylate cyclase specific activity (0.37 nmol/mg protein ·min) was recorded in the 150,000xg supernatant at the onset of idiophasic metabolism. During this growth phase, adenylate cyclase activity also increased in the 1,000xg pellet and was maximally 4.5-fold greater than that in primary phase cultures. No significant cAMP-phosphodiesterase activity could be detected during growht in any of the cell fractions or in the growth medium with either Mn²⁺, Mg²⁺, or Ca²⁺ as added cations. The extracellular cAMP concentration increased logarithmically during primary growth; however, in cultures in idiophasic metabolism cAMP levels remained constant and relatively low. We suggest that excretion into the medium is the principal means by which intracellular cAMP levels are decreased in P. chrysosporium.Abbreviation EB extraction buffer     

Interaction of Triton X-100 with particulate cardiac guanylate cyclase: comparison between Mg2+- and Mn2+-supported enzyme activities in particulate. detergent-solubilized and detergent-insoluble fractions

Guanylate cyclase activity was determined in a 1000g particulate fraction derived from rabbit heart homogenates using Mg²⁺ or Mn²⁺ as sole cation in the presence and absence of Triton X-100. With Mg²⁺, very little guanylate cyclase activity could be detected in the original particulate fraction assayed with or without Triton, or in the particulate fraction treated with varying concentrations of Triton (detergent-treated mixture) prior to enzyme assay. However, the detergent-solubilized supernatants as well as the detergent-insoluble residues (pellets) derived from detergent-treated mixtures possessed appreciable Mg²⁺-supported enzyme activity. With Mn²⁺, significant enzyme activity was detectable in the original particulate fraction assayed without Triton. Much higher activity was seen in particulate fraction assayed with Triton and in detergent-treated mixtures; the supernatants but not the pellets derived from detergent-treated mixtures possessed even greater activity. The sum of enzyme activity in pellet and supernatant fractions greatly exceeded that of the mixture. When the pellets and supernatants derived from detergenttreated mixtures were recombined, measured enzyme activities were similar to those of the original mixture. With Mg²⁺ or Mn²⁺, the specific activity of guanylate cyclase in pellet and supernatant fractions varied considerably depending on the concentration of Triton used for treatment of the particulate fraction; treatment with low concentrations of Triton (0.2–0.7 μmol/mg protein) gave supernatants showing high activity whereas treatment with relatively greater concentrations of the detergent (>0.7 μmol/mg protein) gave pellets showing high activity. The relative distribution of guanylate cyclase in pellet and supernatant fractions expressed as a function of Triton concentration during treatment (of the particulate fraction) showed that 50 to 80% of the recovered enzyme activity remained in supernatants at low detergent concentrations whereas 50 to 80% of the recovered activity resided in the pellets at higher detergent concentrations. Inclusion of excess Triton in the enzyme assay medium did not alter the specific activity profiles and the relative distribution patterns of the cyclase in pellet versus supernatant fractions. The results demonstrate the inherent potential of cardiac particulate guanylate cyclase to utilize Mg²⁺ in catalyzing the synthesis of cyclic GMP. However, it appears that some factor(s) endogenous to the cardiac particulate fraction severely impairs the expression of Mg²⁺-dependent activity; Mn²⁺-dependent activity is also affected by such factor(s) but apparently less severely. Further, the results suggest that previously reported activities of cardiac particulate guanylate cyclase, despite being assayed with Mn²⁺ and in the presence of Triton X-100, represent underestimation of what otherwise appears to be a highly active enzyme system capable of utilizing physiologically relevant divalent cation such as Mg²⁺.     

The HLB dependency for detergent solubilization of hormonally sensitive adenylate cyclase

The HLB dependency for the solubilization of membrane proteins and adenylate cyclase activity from a plasma membrane-enriched fraction from rat liver has been determined. The HLB (hydrophilic/lipophilic/balance) number of a detergent is an empirical measure of its relative hydrophobicity. Detergent HLB numbers vary systematically with the length of the ethylene oxide chain for a homologous series of detergents such as the Triton X series. These detergents have a constant hydrophobic moiety, octylphenyl, and a variable polar portion, polyethoxyethanol. Basal-NaF-epine-phrine-, and glucagon-stimulated adenylate cyclase activities were solubilized in the HLB range of 16.8–17.4. Solubilization was most effective in 0.01 M Tris buffers at pH 7.5 containing 1–5 mM mercaptoethanol, 1 mM MgCl₂, and 0.1% Triton X-305. The detergent to membrane protein ratio used in these studies was 3:1. Criteria for solubilization included lack of sedimentation at 100,000 × g, the absence of particulate material in the supernatant when examined by electron microscopy, and inclusion of hormonally sensitive adenylate cylcase activity in Sephadex G-200 gels. The apparent molecular weight of the solubilized enzyme was approximately 200,000 in the presence of Triton X-305. The solubilized enzyme was stimulated 5-fold by NaF, 7-fold by glucagon, and 20-fold by epinephrine compared to the particulate enzyme used in this study which was stimulated 10-fold, 3,4-fold, and 4-fold by NaF, epinephrine, and glucagon, respectively. The solubilized enzyme is stable for several weeks when stored at ?60° C.     

Effects of phospholipase A and triton x-100 on guanylate cyclase activity in mammary gland homogenates from mice.

The effects of a variety of agents on guanylate cyclase activity were tested in broken cell preparations of mammary glands from midpregnant mice. Of the agents tested, only phospholipase A, triton X-100, and an impure egg lysolecithin preparation enhanced the activity of guanylate cyclase in mammary gland homogenates; other agents, including sodium azide and phospholipase C, and purified egg lysolecithin had no effect. Phospholipase A increased the activity of guanylate cyclase in the 150,000 g pellet fractions of mammary gland homogenates, bud did not consistently enhance guanylate cyclase in the 150,000 g supernatant fractions. Phospholipase A did not appear to enhance guanylate cyclase activity by solublizing the enzyme from the 150,000 g pellet. Triton X-100, in contrast, appeared to act by solubilizing guanylate cyclase from the material present in the 150,000 g pellet. Triton X-100 increased by several fold guanylate cyclase activity in the tissue homogenates and the 150,000 g pellets, but did not consistently enhance enzyme activity in the 150,000 g supernatant. Triton X-100 had no effect on the apparent Km of guanylate cyclase.     

Differential effects of non-ionic detergents on microsomal and sarcolemmal adenylate cyclase in cardiac muscle

1. About 4 and 23% of the homogenate adenylate cyclase activity was recovered in the microsomal and sarcolemmal fractions isolated from guinea-pig heart ventricles. 2. Cardiac microsomal adenylate cyclase activity [basal as well as p[NH]ppG (guanyl-5′-yl imidodiphosphate)- and NaF-stimulated] was increased over 2-fold in the presence of Lubrol-PX (0.01–0.1%). 3. The sarcolemmal enzyme, however, showed concentration-dependent inhibition caused by the detergent under all assay conditions, except when p[NH]ppG was included in the assay. In the latter case, the detergent (0.01–0.02%) caused a modest increase (30–45%) in enzyme activity. 4. Another non-ionic detergent, Triton X-100, also stimulated the microsomal cyclase and inhibited the sarcolemmal enzyme. 5. With either membrane fraction, Lubrol-PX solubilized the enzyme when the detergent/membrane protein ratio was 2.5 (μmol of detergent/mg of protein). 6. The findings with homogenate and a washed particulate fraction resembled those obtained with sarcolemma, and those with isolated sarcoplasmic reticulum resembled those with microsomal preparations. 7. p[NH]ppG, and to some extent NaF, protected the detergent-induced inactivation of the enzyme observed at higher detergent concentrations (0.5% Lubrol-PX and 0.05–0.5% Triton X-100). 8. In the absence of detergents, p[NH]ppG increased the basal enzyme activity about 2-fold in microsomal fractions, but did not appreciably stimulate the sarcolemmal enzyme. Isoproterenol, on the other hand, increased the sarcolemmal enzyme activity (>2-fold) in the presence of p[NH]ppG and caused only moderate stimulation (31%) of the microsomal enzyme under these conditions. 9. These findings support the view that, although the bulk of adenylate cyclase resides in heart sarcolemma (plasma membrane), the microsomal activity cannot be accounted for solely by contamination of the microsomal fraction with sarcolemma, as has been suggested by others [Besch, Jones & Watanabe (1976) Circ. Res. 39, 586–595; Engelhard, Plut & Storm (1976) Biochim. Biophys. Acta 451, 48–61]. Further, the results of this study show that cardiac sarcoplasmic-reticulum membranes possess this enzyme.     

Activation of solubilized liver membrane adenylate cyclase by guanyl nucleotides.

Liver plasma membranes of hypophysectomized rats were purified, treated with 0.1 m Lubrol-PX and centrifuged at 165,000g for 1 h. The detergent solubilized 50% of the membrane protein; adenylate cyclase activity was present in the supernatant fraction. Optimal substrate concentration of the soluble enzyme was 0.32 mm ATP. Basal activity of 25 preparations of the solubilized enzyme ranged from 124 to 39 pmol cyclic AMP/mg protein/10 min. The solubilized enzyme retained the same sensitivity to activation by guanyl nucleotides as was present in the membrane preparation from which it was derived. Relative sensitivity of the solubilized enzyme with 0.1 mm nucleotides or -side was GDP > GTP > GMP > guanosine; GMP-PNP = GMP-PCP > ITP > GTP. GTP, GMP-PCP, GMP-PNP and other nucleotides were hydrolyzed by phosphohydrolases present in liver membranes that were solubilized with Lubrol-PX along with adenylate cyclase. The presence of the ATP regenerating system in the adenylate cyclase assay also aided in maintaining guanyl nucleotide concentrations. The degree of adenylate cyclase activation by guanyl nucleotides was not related to the sparing effects of nucleotides on substrate ATP hydrolysis. These findings demonstrate that activation of adenylate cyclase by nucleotides is a consequence of a nucleotide-enzyme interaction that is independent of membrane integrity.     

Hydroxymethylglutaryl coA reductase (NADPH) in the latex of Hevea brasiliensis

The activity of hydroxymethylglutaryl CoA reductase (NADPH) (EC 1.1.1.34) was studied in the latex of regularly tapped mature trees of Hevea brasiliensis. The reductase activity was found mainly (95% of the total activity) in the pellet fraction (40 000 g) of the centrifuged latex. The enzyme in this fraction had a specific requirement for NADPH as the cofactor and, while not obligatory for activity, was activated by dithiothreitol at the optimum concentration of 2 mM. The pH optimum was found to be 6.6–6.9 in 0.1 M phosphate buffer. Mevalonate and CoA (at 2 mM each) did not affect enzyme activity, while hydroxymethylglutarate (2 mM) was slightly inhibitory. p-Chloromercuribenzoate (1 mM) completely inhibited this enzyme. The reductase activity in the 40 000 g pellet was not easily solubilized either using Triton X-100 or by sonication. The apparent K_m for the washed, membrane-bound enzyme (103 000 g pellet) was 56 μ M (RS-HMG-CoA). Magnesium-ATP (4 mM) inactivated the reductase but this effect was greatly diminished or was absent upon washing the 40 000 g pellet.     

Some effects of triton x-100 on pea etioplasts

When pea etioplast preparations were treated with Triton X-100, the membranes disappeared, the pigments were solubilized, and the organelles appeared to disintegrate. Low speed centrifugation (2000g) of the preparations following treatment with Triton X-100 resulted in a pellet which contained considerable quantities of plastid material. This included RNA polymerase and DNA polymerase activity, much of the DNA, about 30% of the RNA, and 50% of the protein of the washed plastid. The amount of RNA polymerase and DNA polymerase activity associated with the low speed pellet was dependent on the pH during Triton treatment. Significant quantities of the RNA polymerase activity of chloroplasts from spinach, peas, and tobacco were also recovered in the pellet after Triton treatment.     

Adentylate cyclase activity in myocardium of spontaneously hypertensive rat: effect of endogenous factors and solubilization

The isoproterenol- and glucagon-stimulated adenylate cyclase activities in the myocardial membranes of hypertensive rat were consistantly lower as compared with normal controls. Addition of cytosolic fraction (100,000 x$\text{g}$ supernatant) to the particulate preparation had an additive effect for glucagon and Gpp(NH)p stimulated enzyme activity and a synergistic effect for isoproterenol stimulation. Cytosolic fraction of normal control animals did not bring the adenylate cyclase activity in SHR equivalent to the control values. The basal and F^?-stimulated enzyme activity of solubilized adenylate cyclase was reduced by about 30% in SHR as compared with WKY, which could be due to a decrease in the actual amount of adenylate cyclase in the myocardium of SHR.     

Regulation of adenylate cyclase from Leucophaea maderae by calcium,calmodulin and forskolin

1. Adenylate cyclase was assayed in homogenates ofhindgut tissue from Leucophaea maderae (L.). The 10,000 g supernatant enzyme was stimulated by calmodulin.2. Trifluoperazine inhibited calmodulin-stimulated adenylate cyclase activity.3. Elevated calcium levels (> 100 μM) inhibited natural and calmodulin-stimulated enzymic activity.4. Forskolin (1 mM) stimulated adenylate cyclase activity by approximately 10-fold.     

Increase of proton electrochemical potential and ATP synthesis in rat liver mitochondria irradiated in vitro by helium-neon laser

Particulate adenylate cyclase (AC) and guanylate cyclase (GC) activities localized in the ciliary membrane from Paramecium were solubilized by a two-step procedure using the detergents Brij 56 and Lubrol PX. The enzymes remained in the supernatant after a 100 000 × g centrifugation. Upon gel chromatography, AC and GC were almost completely separated proving that each enzyme is a distinct molecular entity. Solubilization of GC was achieved with the calmodulin subunit remaining firmly attached to the catalytic part. Antibodies against calmodulin inhibited the enzyme as did La³⁺ and EGTA. AC activity appeared to be regulated specifically by K⁺, enzyme activity being enhanced up to 100% by 15 mM K⁺. Na⁺ and Li⁺ were inactive.     

Guanylate cyclase in the accessory gland of the cricket, Acheta domesticus

Guanylate cyclase (E.C. 4.6.1.2.) was investigated in the accessory reproductive gland of the male house cricket, Acheta domesticus, which is known to accumulate exceptionally high levels of guanosine 3′,5′-cyclic monophosphate (cyclic GMP). Accessory gland guanylate cyclase activity was linear with time for at least one hour, and with enzyme concentration to about 5 mg soluble protein per ml. Activity was dependent on Mn²⁺ and was maximal at pH 7.3 to 8.0. Sodium fluoride had no effect on activity, but sodium azide was slightly stimulatory. About 80% of the activity was sedimentable at 16,000 g, and both soluble and particulate activities were increased slightly in the presence of Triton X-100. Kinetic analysis indicated half-maximal velocity at 85 μM GTP in the presence of excess Mn²⁺, and reciprocal plots were concave upward. Changes in activity during maturation of the gland were small, and did not provide evidence for a regulatory role of guanylate cyclase in the accumulation of accessory gland cyclic GMP. The regulation and rôle of cyclic GMP in the accessory gland are discussed.     

The size of adenylate cyclase and guanylate cyclase from the rat renal medulla

The size distribution of adenylate cyclase from the rate renal medulla solubilized with the nonionic detergents Triton X-100 and Lubrol PX was determined by gel filtration and by centrifugation in sucrose density gradients made up in H₂O or D₂O. The physical parameters of the predominant from in Triton X-100 are ²20,w, 5.9 S; Stokes radius, 62 A; partial specific volume (v ), 0.74 ml/g; mass, 159,000 daltons; f/f₀, 1.6; axial ratio (prolate ellipsoid), 11. For the minor form the values are : ²20,w, 3.0; Stokes radius, 28 A; mass, 38,000 daltons; f/f₀, 1.2. The corresponding values determined in Lubrol PX are similar. The value of v for the enzyme indicates that it binds less than 0.2 mg detergent/mg protein. Since interactions with detergents probably substitute for interactions with lipids and hydrophobic amino acid side chains, these findings suggest that no more than 5% of the surface of adenylate cyclase is involved in hydrophobic interactions with other membrance components. Thus, most of the mass of the enzyme is not deeply embedded in the lipid bilayer of the plasma membrance. Similar studies have been performed on the soluble guanylate cyclase of the rate renal medulla. In the absence of detergent, the molecular properties of this enzyme are: ^s20,w, 6.3 S; Stokes radius, 54 A, v , 0.75 ml/g; mass, 154,000 daltons f/f₀, 1.4; axial ratio, 7. The addition of 0.1% Lubrol PX to this soluble enzyme increases its activity two- to fourfold and changes the physical properties to : ^s20,w, 5.5 S; Stokes radius, 62 A; v , 0.74 ml/g; mass, 148,000 daltons; f/f₀, 1.6; axial ratio, 11. These results show that Lubrol PX activates the enzyme by causing a conformational change with unfolding on the polypeptide chain. Guanylate cyclase from the particulate cell fraction can be solubilized with Lubrol PX but has properties quite different from those of the enzyme in the soluble cell fraction. It is a heterogeneous aggregrate with ^s20,w, 10 S; Stokes radius, 65 A; mass about 300,000 daltons. The conditions which solubilize guanylate cyclase also solubilize adenylate cyclase and the two activities can be separated on the same sucrose gradient.     

Guanosine triphosphate and colchicine affect the activity and the polymeric state of acetyl-CoA carboxylase

Acetyl-CoA carboxylase was purified 300-fold from rat liver, in the absence of added citrate, by precipitation from an 18,000g supernatant in the presence of Triton X-100 at 105,000g and 20 °C, followed by chromatography on phosphocellulose. Acetyl-CoA carboxylase activity in this preparation was activated by preincubation with GTP (0.1–2.0 mm) and with citrate (20 mm). Colchicine (10^?6–10^?3m) inhibited enzyme activity and counteracted the effects of GTP and citrate. Sucrose density gradient centrifugation demonstrated that GTP and citrate preincubation promoted the formation of the polymeric, active enzyme, while colchicine engendered disassembly. Preincubation of the purified acetyl-CoA carboxylase at 4 °C caused inactivation and disassembly, which was countered by preincubation at 37 °C in the presence of GTP or citrate. These results suggest that GTP, like citrate, activates acetyl-CoA carboxylase by enhancing the conversion of the protomeric form of the enzyme to its more active, polymeric state.     

Guanylate cyclase. Subcellular distribution in cardiac muscle, skeletal muscle, cerebral cortex and liver.

1. Guanylate cyclase of every fraction studied showed an absolute requirement for Mn2+ ions for optimal activity; with Mg2+ or Ca2+ reaction was barely detectable. Triton X-100 stimulated the particulate enzyme much more than the supernatant enzyme and solubilized the particulate-enzyme activity. 2. Substantial amounts of guanylate cyclase were recovered with the washed particulate fractions of cardiac muscle (63-98%), skeletal muscle (77-93%), cerebral cortex (62-88%) and liver (60-75%) of various species. The supernatants of these tissues contained 7-38% of total activities. In frog heart, the bulk of guanylate cyclase was present in the supernatant fluid. 3. Plasma-membrane fractions contained 26, 21, 22 and 40% respectively of the total homogenate guanylate cyclase activities present in skeletal muscle (rabbit), cardiac muscle (guinea pig), liver (rat) and cerebral cortex (rat). In each case, the specific activity of this enzyme in plasma membranes showed a five- to ten-fold enrichment when compared with homogenate specific activity. 4. These results suggest that guanylate cyclase, like adenylate cyclase, and ouabain-sensitive Na+ + K+-dependent ATPase (adenosine triphosphatase), is associated with the surface membranes of cardiac muscle, skeletal muscle, liver and cerebral cortex; however, considerable activities are also present in the supernatant fractions of these tissues which contain very little adenylate cyclase or ouabain-sensitive Na+ + K+-dependent ATPase activities.     

Solubilization,stabilization and isoelectric focusing of human liver neuraminidase activity

Homogenate preparations of human liver have been prepared and over 75% of the particulate neuraminidase activity (which comprises approx. 90% of the total activity) has been solubilized using 0.85% (w/v) Triton X-100 in 25 mM phosphate buffer (pH 6.8). The solubilized neuraminidase activity is extremely labile, but can be stabilized for at least 4 weeks at 2–4°C, using 10 mM N-acetylneuraminic acid. Kinetic characterization of homogenate and solubilized supernatant fluid neuraminidase activities indicated comparable pH optimum curves (maximum activity at pH 4.5–4.7) and apparent K_m values (0.2–0.4 mM) for the synthetic fluorometric substrate $\text{4-}\text{methylbelliferyl}\text{-α-}\text{D}\text{-N-}\text{acetylneuraminic}$ acid. Isoelectric focusing has been performed on human liver homogenates and Triton X-100-solubilized neuraminidase activities, and the presence of several forms (4–6) with isoelectric points (pI values) between 4.4 and 5.2 has been demonstrated in both preparations. The similar kinetic and isoelectric focusing properties of the two preparations suggest that the solubilized enzyme activity is representative of the homogenate activity and that the solubilized enzyme is suitable for purification purposes.     

ATPase activity of Tetrahymena cilia before and after extraction of dynein

Cilia from Tetrahymena pyriformis were extracted twice with Tris-EDTA. The first extraction increased the total ATPase activity by about one-third. No increase in activity occurred as a result of the second extraction, but 40% of the original ATPase activity remained in the pellet. The activity remaining in the pellet differed in its substrate specificity, its thermostability, and its sensitivity to monovalent cation chlorides from the solubilized dynein. Several of the properties of the ATPase activity of whole cilia differed from those computed for a mixture of 40% pellet ATPase + 60% solubilized dynein ATPase. From these differences it was deduced that dynein in situ is more thermostable than is solubilized dynein and, in contrast to solubilized dynein, is slightly inhibited by KCl, NaCl, LiCl, and NH₄Cl. The increase in total activity upon solubilization of the dynein and the changes in thermostability and in sensitivity to monovalent cations indicates that dynein ATPase in situ is modified by interaction with other components of the axonemal bend generating system.The pellet remaining after extraction of dynein by two dialyses against Tris-EDTA was treated with 0.4% Triton X-100 to solubilize ciliary membranes. Less than half of the ATPase activity was solubilized by this treatment. The possibility that the activity remaining in the Tris-EDTA- and Triton X-100-extracted residue represents an additional ATPase of cilia is discussed.     

Guanine-deaminase activity in rat brain and liver

1. Guanine deaminase in rat brain and liver was distributed among all the subcellular fractions: nuclei, `heavy' mitochondria, `light' mitochondria, microsomes and the supernatant fluid. The greater part of the activity passed into the soluble fraction. Among the particulate components, the `light' mitochondria constituted the richest fraction. 2. The sum of the enzymic activities of the component fractions obtained on differential centrifugation was considerably greater than the activity of guanine deaminase in the whole homogenate. 3. The `heavy'-mitochondrial fraction had a powerful inhibitory effect on the guanine-deaminase activity of the supernatant fraction. 4. All the sedimented fractions, except the microsomes, gave rise to higher guanine-deaminase activity on treatment with Triton X-100. 5. The inhibitory capacity of the `heavy' mitochondria increased on treatment with Triton X-100; the detergent-treated nuclear fraction also brought about inhibition of the 5000g supernatant. 6. Guanine-deaminase inhibitor from the `heavy' mitochondria was solubilized by high-speed grinding of the particles, followed by treatment with Triton X-100. The inhibitor appeared to be protein in nature, since it was precipitated by trichloroacetic acid and by half-saturation with ammonium sulphate, and was non-diffusible. It was inactivated by heating at 50° for 5min. 7. It is possible that the guanine deaminase associated with particles differs from the soluble enzyme in its response to inhibitor.