The Activity And Mechanism Of Phosphatase Inhibited By Arsenic In Different PH

Arsenic(As)is one of the heavy metals polluted soil,it's toxicity depended by speciation and environmental conditions such as pH.Phosphatase can catalyzes phosphorus cycle in soil.In this study,the correlation between arsenic and alkaline phosphatase(ALP)with different states(free,montmorillonite immobilized,soil)at pH 7.0,8.0,9.0 and 10.0 was investigated by kinetics and thermodynamics methods.In addition,the correlation between arsenic and acid phosphatase(ACP)activities in three soil samples(Hunan Red Soil,Jilin Black Soil,Chongqing Purple Soil)at pH 4.0,5.0,6.0 and 7.0 was explored by kinetics method.The results were shown as follows:(1)ALP activity was significantlyinhibited by arsenic in the tested pH and the inhibition ratio was enhanced with the increasing of arsenic concentration and pH.The inhibition varied in different states and showed as: F-ALP > M-ALP > S-ALP.The relationship between ALP activities and arsenic concentration could be fitted well by the full inhibition model y=c/(1+b×i).The calculated ecological dose(ED10)were lower than the Environmental quality standard for soils(1995)(the third grade),which means ALP activity could be a good bioindicator for the arsenic pollution inthe range of pH 7.0 to 10.0.(2)For F-ALP,the increase of arsenic concentration could increase Km and decrease Vmax and Vmax/Km.For M-ALP and S-ALP,Vmax had no significant change and Vmax/Km decreased as arsenic concentration increased.The state of ALP had an effect on the inhibition mechanism of arsenic on ALP.There was a competitive inhibition combined with linear mixed inhibition mechanism of arsenic on F-ALP,while the mechanism was completecompetitive inhibition of arsenic on M-ALP and S-ALP.In the range of pH 7.0~10.0,pH did not affectthe arsenicinhibitory mechanism on ALP,but had an influence onthe kinetic parameters of ALP under arsenic stress.With the increasing of p H,the affinity of arsenic and enzyme increased.The ALP kinetic parameters Km,Vmax and Vmax/Km increased,leading to thedecrease of ecological dose and the increase of arsenictoxicity.(3)In the thermodynamics study,the average temperature coefficient(Q10)of ALP is 0.709~4.396.Q10 was increased with the increasing of arsenic concentration.Pre-esponential factor(A)and activation energy(Ea)increased with the increasing of arsenic concentration and the decreasing of pH.A and Ea of arsenic on ALP activity were varied as existence states: S-ALP > F-ALP > F-ALP.In the tested pH,the thermodynamic parameters of three states of alkaline phosphatase were shown as:(35)G* > 0,(35)H* > 0,(35)S* < 0,indicatingthat the reaction of ALP under arsenic pollution was a non-spontaneous endothermic reactionand the order of the system was improved inthe reaction process.Arsenic could enhance Ea,(35)G*,(35)H*,and(35)S*but reduce system order.High pH decreased Ea,(35)G*,(35)H*,and(35)S* but increased system order.As shown by apparent ALP activity,arsenic pollution could lead to the increase of Ea and ?H*,which means that the inhibition belongs to enthalpy control process.(4)Arsenic significantly inhibited the activity of ACP in the tested pH.The toxicity of arsenic increased with the decreasing of pH and the increasing of arsenic concentration.The relationship between ACP activities and arsenic concentration could be describedwell bythe partial inhibition model y=c(1+ai)/(1+bi).ED10 were lower than the first standard of Environmental quality standard for soils(1995),which means ACP activity could be a good bioindicator for the arsenic pollution in the range of pH 4.0 to 7.0.(5)The soil type and pH had effects on the inhibition mechanism of arsenic on ACP.For Hunan Red Soil and Chongqing Purple soil samples,Km increased and Vmax and Vmax/Km decreased with the increasing of arsenic concentration.There was a competitive inhibition combined with linear mixed inhibition mechanism of arsenic on ACP in Hunan Red soil sample.For Chongqing Purple Soil sample,the mechanism was competitive inhibition at pH 4.0 and 5.0,while exhibited as competitive inhibition combined with linear mixed inhibition at the other two pH.For Jilin Black Soil sample,Km had no significant change,however,Vmax and Vmax/Km decreased.The mechanism was noncompetitive inhibition mechanism of arsenic at pH 4.0 and 6.0,and linear mixed inhibition mechanism with the similar ratio of competitive and noncompetitive inhibition at pH 5.0,and competitive inhibition at pH 7.0.In summary,this paper obtains the following conclusion: Arsenic significantly inhibited the activity of ACP and ALP,and the the inhibition of ALP belongs to enthalpy control process.pH had effects on the inhibition mechanism of arsenic on ACP,but pH did not affect the arsenicinhibitory mechanism on ALP.ACP and ALP activities could be good bioindicators for the arsenic pollution.