(Choices B and C) Intracellular cAMP and cGMP concentrations increase during activation of adenylate or guanylate cyclase second messenger systems, respectively. Levels can also increase following cyclic nucleotide
phosphodiesterase inhibition, as seen on exposure to sildenafil, which selectively inhibits cGMP phosphodiesterease and results in smooth muscle relaxation in blood vessels.
(Choice D) Intracellular Cl· concentration increases slightly after inhibitory neurotransmitters (eg, GABA, glycine) act on the neuron to increase Cl· membrane conductance (hyperpolarization).
(Choice E) The intracellular concentration of mRNA increases during cellular states of elevated protein synthesis (eg, during cell division).
(Choice F) Nitric oxide (NO) is a paracrine signaling molecule with a lifetime of a few seconds. It can freely cross cell membranes and functions as a critical component of endothelium-mediated vasodilation. NO is synthesized from arginine and 02 by the enzyme NO-synthase.

(Choices Band C) Histones H3 and H4 are components of the nucleosome core.
(Choice D) Small nuclear ribonucleoproteins help splice out intrans from pre-mRNA, forming mature mRNA.
(Choice E) During DNA replication, topoisomerase relieves the tension created during DNA strand unwinding by introducing negative supercoils into the DNA. It does not promote chromatin compaction.
(Choice F) Ubiquitin is a small protein present in the cytoplasm and nucleus of all eukaryotes. It is typically covalently attached to various intracellular proteins to signal for their degradation by the proteasome (ubiquitinproteasome pathway).

(Choices A, D, E, and G) lsocitrate dehydrogenase, succinate thiokinase, malate dehydrogenase, and fumarase are enzymes that participate in the TCA cycle but do not use FAD or FMN as cofactors.
(Choice B) Reduced glutathione is an antioxidant that minimizes oxidative damage in many cells. Glutathione reductase regenerates reduced glutathione using nicotinamide adenine
dinucleotide phosphate (NAD PH) as an electron donor and FAD as a cofactor. Although glucose-6-p hosphate dehydrogenase (G6PD) is the rate-limiting enzyme in the pentose phosphate pathway and supplies the necessary NADP H, it does not use FAD as a cofactor.
(Choice C) HMG-CoA reductase is the rate-limiting enzyme in the cholesterol synthesis pathway. FMN and FAD are not used as cofactors.