Question: What is the most common isotope of the hydrogen atom?
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Primary hydrogen isotope effects.
Primary hydrogen kinetic isotope effects refer to cases in which a bond to the isotopically labeled hydrogen is formed or broken in the rate-determining step of the reaction.
These are the most commonly measured kinetic isotope effects, and much of the previously covered theory refers to primary kinetic isotope effects.
When there is adequate evidence that transfer of the labeled hydrogen occurs in the rate-determining step of a reaction, if a fairly large kinetic isotope effect is observed, e.g.
kH/kD of at least 5-6 or kH/kT about 10-13 at room temperature, it is quite likely that the hydrogen transfer is linear and that the hydrogen is fairly symmetrically located in the transition state.
It is usually not possible to make comments about tunneling contributions to the observed isotope effect unless the effect is very large.
If the primary kinetic isotope effect is not as large, it is generally considered to be indicative of a significant contribution from heavy-atom motion to the reaction coordinate, although it may also mean that hydrogen transfer follows a nonlinear pathway. 
Deuterated dimethylformamide ((CD3)2NCOD), is a form (called an isotopologue) of DMF ((CH3)2NCOH)in which the hydrogen atom ("H") is replaced with deuterium (heavy hydrogen) isotope ("D").
Deuterated DMF is a relatively uncommon solvent used in NMR spectroscopy. 
Deuterated ethanol (C2D5OD), is a form (called an isotopologue) of ethanol (C2H5OH) in which the hydrogen atom ("H") is replaced with deuterium (heavy hydrogen) isotope ("D").
Deuterated ethanol is an uncommon solvent used in NMR spectroscopy. 
An important goal of measuring enzyme kinetics is to determine the chemical mechanism of an enzyme reaction, i.e., the sequence of chemical steps that transform substrate into product.
The kinetic approaches discussed above will show at what rates intermediates are formed and inter-converted, but they cannot identify exactly what these intermediates are.
Kinetic measurements taken under various solution conditions or on slightly modified enzymes or substrates often shed light on this chemical mechanism, as they reveal the rate-determining step or intermediates in the reaction.
For example, the breaking of a covalent bond to a hydrogen atom is a common rate-determining step.
Which of the possible hydrogen transfers is rate determining can be shown by measuring the kinetic effects of substituting each hydrogen by deuterium, its stable isotope.
The rate will change when the critical hydrogen is replaced, due to a primary kinetic isotope effect, which occurs because bonds to deuterium are harder to break than bonds to hydrogen.
It is also possible to measure similar effects with other isotope substitutions, such as 13C/12C and 18O/16O, but these effects are more subtle.
Isotopes can also be used to reveal the fate of various parts of the substrate molecules in the final products.
For example, it is sometimes difficult to discern the origin of an oxygen atom in the final product; since it may have come from water or from part of the substrate.
This may be determined by systematically substituting oxygen's stable isotope 18O into the various molecules that participate in the reaction and checking for the isotope in the product.
The chemical mechanism can also be elucidated by examining the kinetics and isotope effects under different pH conditions, by altering the metal ions or other bound cofactors, by site-directed mutagenesis of conserved amino acid residues, or by studying the behaviour of the enzyme in the presence of analogues of the substrate(s). 
The most abundant isotope, hydrogen-1, protium, or light hydrogen, contains no neutrons; other isotopes of hydrogen, such as deuterium or tritium, contain one or more neutrons.
The formulas below are valid for all three isotopes of hydrogen, but slightly different values of the Rydberg constant (correction formula given below) must be used for each hydrogen isotope.