There are a few problems I can see. There could be more, but this should get you started:
Add ‘par’ to the Integrand function arguments:
Integrand = @(par,x) ((par(3)/(par(5) ...
The Integral function then becomes:
Integral = @(par,x) integral(@(x) Integrand(par,x),0,x);
I suggest that you name your initial parameter estimates ‘par0’ (or something other than ‘par’) to avoid confusion:
par0 = [1e-9,1e-5,1,10000,1,1];
est_par = nlinfit(A(:,1),A(:,2),Integral,par0)
The ‘est_par’ assignment are the parameters estimated by nlinfit.
I can’t test your code (so no guarantees), but these changes should at least allow it to run.