Studies of several species reveal a wide spectrum of alterations in mitochondria, mainly during the aging. My interest is in investigating the changes of these alterations in plasma.
I suggest TEM for counting and morphometric analysis of mitochondria. second step IHC for cytochromes
Your question is not clear enough. Do you want to measure the activity of mitochondria or are you looking for a mitochondrial activity biomarker in plasma? In case of former, the answer is easy - measure the respiration (oxygen consumption rate) in purified mitochondria using seahorse or equivalent machinery. As for the biomarker - that's a trickier question to answer. There is no consensus metabolite to rule them all. AcCoA, AMP/ATP and NAD/NADH ratios come to mind. You'll have to decide yourself.
You can also measure specific enzymatic activity of the complexes of the electron transport system (easier and cheaper). I would suggest CI+CIII (falsely called the ETS enzymatic activity) and/or CIV activity. Additionally, you canmeasure the citrate synthase activity which is a good marker of mito density...
Yes, that works too. But in essence, it's probing the NADH consumption rate (see NAD/NADH ratio above). Citrate synthase is an easy one, so I agree - start there.
A word of caution on use of TEM or IHC - neither one measures the activity, just quantifies the number of mitochondria. You could have abundance of mitochondria, but they can have low activity due to variety of factors.
Presence of mitochondrial membrane bound proteins, ionic pumps, ration of NAD and NADPH, F0F1ATPase activity, level of ubiquinone functions and cytochromes p450 are the best measures to predict mitochondrial functions. Number of ATP released and rate of catabolism also display mitochondrial activity.
For activity, you may want to look at O2 consumption as mentioned by another comment. For quantifying functional mitochondria, I would recommend using TMRE and mitotracker/JC-1 dyes. TMRE is a dye that is bright under high mitochondiral membrane potential (read: healthy mitochondria) but dim under low mitochondrial membrane potential (read: unhealthy/dying mitochondria). Performing a ratio of TMRE to mitotracker/JC-1 (either will work) will allow you to quantify mitochondrial membrane potential for the total mitochondrial mass.
The linked paper provides a really good summary of different assays to asses mitochondria function and health.Article Cytometric Assessment of Mitochondria Using Fluorescent Probes
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