Do these help?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4814709/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4814709/table/tab1/

interesting excerpts:

"Metabolic differences between mitochondria of WAT and BAT are associated with specific morphological characteristic of mitochondria in the brown adipocytes. These mitochondria are apparently more numerous and bigger in size and contain more cristae than mitochondria in white adipocytes. In addition, content of the heme cofactors in the mitochondrial enzyme cytochrome oxidase gives the tissue the brown macroscopic colour [16, 76]. Compared to BAT, WAT has fewer mitochondria, mostly undetectable expression of UCP1, and lower expression levels of the fatty acid β-oxidation-related enzyme, acyl CoA dehydrogenase, suggesting that the intensity of β-oxidation in WAT is lower than in BAT [53, 56, 77, 78]. As in other tissues, mitochondria represent the main source of ATP in the white fat. White fat mitochondria are well equipped for oxidative phosphorylation, with pyruvate serving as a main source of energy for ATP synthesis. Due to low activity of carnitine palmitoyltransferase 1 in the inner mitochondrial membrane, oxidation of fatty acids is relatively slow and fatty acids are directed towards esterification, unless the transferase is activated by leptin [79]."...

...

"This process, termed nonshivering thermogenesis, is specific function of BAT/bAT and is particularly important during hibernation and for small animals and infants who have greater demands on thermogenesis due to a large surface-to-volume ratio [57]. The molecular substrate of this unique function is a protein containing three similar repeats of about 100 amino acids coded by nuclear genes and inserted into the inner mitochondrial membrane [58]. As the major role of the protein is proton translocation resulting in uncoupling of the electron-transporting system from ATP synthesis in the mitochondria, it was named uncoupling protein (UCP) [59].

Uncoupling proteins belong to a family of mitochondrial carrier proteins that are present in the mitochondrial inner membrane. Mammals express five UCP homologues (UCP1 also named thermogenin), UCP2, UCP3, UCP4, and UCP5, also known as brain mitochondrial carrier protein 1 (BMCP1) [60]. UCP1 is expressed almost exclusively in fully differentiated BAT cells [17], although some findings suggest that UCP1 can be detected also in other tissues including uterine smooth muscle and even WAT, where induction of uncoupling protein expression is associated with acquiring of brown fat features [61, 62]."...

Quick guess (from a quick reading by a non-expert): I'd expect a really small part (in average) unless in conditions where they may be:

- stressed to synthesize ATP,

- or idle enough (not that we could ever be like reptiles) to lower UCP1 usage

- or when UCP1 process doesn't function correctly (mutation?)

Thanks, George!

Thank you, George for the information.

BAT mitochondria are permanently uncoupled. They contain UCP i. e. UnCouplerProtein. Their function is to generate heat to maintain body temperature. Hence rather than synthesizing ATP the energy is used in the form of heat. For more information look up D. G. Nicolls (on PUBMED) who has done poineering anf authoritative work on BAT mitochondria.

I think I found what I was looking for: In his recent (2017) review, Nicholls mentions, "Under thermogenic conditions, BAT mitochondria must play a balancing act by increasing respiration while at the same time maintaining sufficient cytosolic adenine

nucleotide phosphorylation potential for the cell's housekeeping

processes, not least the provision of ATP for fatty acid activation by

acylCoA synthetase." So, thank you very much, Surenda!

btw, came across this db entry that looks interesting:

http://www.uniprot.org/uniprot/P25874

- and the following from

http://www.jbc.org/content/285/29/21961.full

BAT is characterized by an exceptionally high presence of mitochondria that possess only a minor amount of ATP synthase to convert nutritional into cellular energy equivalents (6). Instead, brown adipose tissue mitochondria are specialized by a high content of mitochondrial uncoupling protein 1 (UCP1), up to 8% of total mitochondrial protein (7), which generates a proton leak in the mitochondrial inner membrane dissipating proton motive force as heat (8). UCP1 can be inhibited by physiological concentrations of purine nucleoside di- and triphosphates before free fatty acids can overcome inhibition (8).

Cold exposure stimulates lipolysis in BAT, thereby activating UCP1. Long term cold acclimation further increases the thermogenic capacity of BAT by enhanced UCP1 expression and recruitment of oxidative capacity (9). The importance of UCP1 for brown adipose tissue-derived heat production has been confirmed in UCP1-ablated mice, which are unable to defend their body temperature when acutely exposed to the cold (4 °C) (10). Successive acclimation improves the cold tolerance of UCP1-ablated mice, suggesting that other sources of heat production partially compensate for the loss of functional BAT, namely muscle shivering (11, 12) and accelerated metabolic flux in white adipose tissue (13, 14). These alternative thermogenic mechanisms appear to be less efficient and are disadvantageous because the median survival rate of UCP1-ablated mice decreases in the cold from >24 to ∼13 weeks as compared with wild type mice, raising the question about the advantages of UCP1-mediated thermogenesis.

- The synopsis below btw, got me a bit confused on whether ATP is blocked or converted:

https://en.wikipedia.org/wiki/Thermogenin

UCPs are transmembrane proteins that decrease the proton gradient generated in oxidative phosphorylation. They do this by increasing the permeability of the inner mitochondrial membrane, allowing protons that have been pumped into the intermembrane space to return to the mitochondrial matrix. UCP1-mediated heat generation in brown fat uncouples the respiratory chain, allowing for fast substrate oxidation with a low rate of ATP production. UCP1 is related to other mitochondrial metabolite transporters such as the adenine nucleotide translocator, a proton channel in the mitochondrial inner membrane that permits the translocation of protons from the mitochondrial intermembrane space to the mitochondrial matrix. UCP1 is restricted to brown adipose tissue, where it provides a mechanism for the enormous heat-generating capacity of the tissue.

UCP1 is activated in the brown fat cell by fatty acids and inhibited by nucleotides. Fatty acids cause the following signaling cascade: Sympathetic nervous system terminals release Norepinephrine onto a Beta-3 adrenergic receptor on the plasma membrane. This activates adenylyl cyclase, which catalyses the conversion of ATP to cyclic AMP (cAMP). cAMP activates protein kinase A, causing its active C subunits to be freed from its regulatory R subunits. Active protein kinase A, in turn, phosphorylates triacylglycerol lipase, thereby activating it. The lipase converts triacylglycerols into free fatty acids, which activate UCP1, overriding the inhibition caused by purine nucleotides (GDP and ADP). At the termination of thermogenesis, the mitochondria oxidize away the residual fatty acids, UCP1 inactivates and the cell resumes its normal energy-conserving mode.

BAT mitochondria are permanently uncoupled. They contain UCP i. e. UnCouplerProtein. Their function is to generate heat to maintain body temperature. Hence rather than synthesizing ATP the energy is used in the form of heat.