In a perfect world, research to understand how genetic mutations and polymorphisms (e.g., SNPs) alter enzyme expression or activity would be robust and reproducible. Unfortunately, very few enzymes fall into this reasonably well researched category. Those are enzymes involved in drug metabolism and, even then, it is drug metabolites being studied, not the substrate(s) the enzymes evolved to process.
In the simplest of terms, enzyme kinetics are determined in a lab setting as a study of the reaction rate of a chemical process at a given temperature and concentration of the substrate being studied. Lab techniques utilized include high performance liquid chromatography and, more recently, mass spectrophotometry. This research characterizes the reference rates for wild type/normal genes and a stated substrate (usually quoted as a Km value at a given temperature and concentration).
When it comes to the study of mutations and how they affect rates of reactions, there is no common gold standard for the method employed. Mutations can affect enzymes in many ways: they can increase or decrease the transcription and/or expression of the enzyme made by the protein. In this case, the rate or kinetics of the reaction is the same as wild type, but there is much less (or more) of the enzyme to do the work. In addition to changing transcription rates, mutations can impact mRNA processing, stability or splicing. Another possibility is that the mutation may change the amino acid sequence that affects the active site of the enzyme which can either enhance (speed up) or hinder (slow down) binding of substrate.
Given this range of possible sequelae, researchers have used many methods to study mutation effects on enzyme activity. These include e coli with recombinant DNA, human cell lines engineered to include the mutation, lab animals bred to have the mutation, or measuring the activity of enzyme in patients' cells that carry the mutation (e.g.: lymphocytes, liver cells) in laboratory setting or even in real life. It is generally felt that research involving humans is superior to extrapolation from E. coli or other mammalian data, but unfortunately information is lacking in this regard for many enzymes.
As for the labeling signified by " ih " in the PDF, this refers to conflicting results in the literature: perhaps an in vitro human cell line study of an enzyme clearly resulted in reduced activity, but another group of researchers tried to replicate the same study and arrived at a different result. Or a piece of research is deemed inappropriate by some authorities as it utilized outdated or otherwise unproven laboratory methods or poor study design. Moreover, in vitro results may not hold when conducted in vivo as there are more confounding factors like epigenetics or gene-gene interactions to consider. Unfortunately, science is not a smooth progression forward in intellectual understanding.
As with any research, it is important to put results into context and look for supporting or conflicting evidence in the patient.
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